This is a write up I initially did for a photography forum I participate in. Several members on the forum found it useful so I thought I’d add it here.
This is purely an introduction to basic colour management. It’s not intended to be an in depth treatise on the subject. This is also not intended to be a highly technical discussion of the subject because sometimes I think we get too caught up in the technical minutiae and lose sight of practicality. This is based on what I’ve learned through experience and from others over time.
If there are any mistakes in what follows, I’m certainly happy to hear about them and you can use the Contact link at the bottom of the page to send me a message.
What is Colour Management (or Color if you prefer)
In very simple terms, colour management is a process undertaken in a digital imaging workflow to ensure colour consistency from start to finish; that is, from camera/scanner (input) through editing through output (print, web or other).
Why is Colour Management Important?
Why do we need colour management? Red is red, green is green and blue is blue, right? Well, not really. Digital cameras, scanners and printers all render colours a bit differently. The could impart a slight warm or cool tone or there could be significant hue shifts. The biggest impact, for most people, is seen in printing. Printers and inks are getting better but you’ll still likely see inaccurate results without a properly colour managed workflow.
How do I Achieve Good Colour Management?
Colour management is an active process. It’s not something that’s going to happen on its own. We need to be involved and take control of our workflow.
The key elements in the process are calibration and profiling. What does that mean? What is calibration? What is profiling?
Like any other instrument, calibration is a process of zeroing out or removing any built in biases. As mentioned above, digital cameras, scanners and monitors all may have; do have, built in (not necessarily intentionally) biases in the way they read and render colour. The software that’s used to convert analogue signals in cameras and scanners to a digital image is good but it’s not perfect. Similarly monitors have biases that need to be filtered out. The calibration process does exactly this.
Very simply, when calibrating a device, a patch of colour blocks called an IT8 target is used. The target is captured or printed by the device and the colour values rendered by the device are measured. These colour values are then compared to a reference table that contains the actual, correct RGB colour values of the colour patches. The calibration software compares the proper values to the measured values and creates a set of differences. This set of differences becomes the basis of the ICC profile. The ICC profile gets stored on your hard drive and is available to be used in editing programs, printing and by your graphics card. When the ICC profile is applied, the colour values in the image are compared with the ICC profile and the colours we see on screen or in a print are altered (corrected) for the biases in the device.
Calibrating a monitor is very simple. In the calibration process, you determine what reference points you’re going to use for colour (e.g., Gamma and Colour Temperature) and make adjustments to the monitor’s controls (Brightness, Contrast, Colour Temp, RGB) to meet those parameters. The calibration device you place on the monitor screen reads a series of colour patches. It notes differences between the colour and brightness of the patches being produced by the monitor and compares those to the target values. You make adjustments to the monitor controls until the variances between the actual and target values is brought to within an acceptable range. These settings get saved in the ICC profile for the monitor and are used by the graphics card in rendering colour on screen.
The process for profiling printing paper and ink combinations is much the same. A standard colour block target is printed. A device called a spectrophotometer is used to read the colour values of the printed target. The colour values are compared to a reference source which contains the actual colour values of the block pattern. A set of differences is created and stored as an ICC profile. That profile is then used in the printing process to correct for differences in colour rendered by the printer with the paper/ink combination being used. Even with the same inks, different papers render colour differently so a profile is needed for each paper you plan to use in your printing.
The process of calibrating your monitor and profiling your printer can be done with either software alone or a combination of hardware and software. I’ll discuss this in greater detail below.
In my opinion, calibrating a digital camera isn’t overly important. If you shoot RAW, you can make any necessary corrections to colours in the RAW conversion process. If you shoot JPEG, the image is already tagged with a colour space profile that’s going to be pretty accurate. On top of that, changing colours in a JPEG file is a destructive process and it’s more difficult to adjust colour in a JPEG file than a RAW file. I’ve looked at differences between images from my cameras with and without a profile and the differences are so small it’s not worth worrying about. My Canon 5D seems to render marginally warmer; which I actually like, and I shoot RAW so have full flexibility to adjust colours and colour temperature in the file nondestructively. The colour from my Canon 40D seems pretty much bang on when comparing images with and without a profile. If you want to calibrate your camera, the major companies that produce calibration hadware like ColorVision and XRite have tools for this purpose.
Calibrating a Scanner
When you calibrate a scanner, a flatbed can be done with either a printed target or a film target. Some people will say you should do it for both. With a dedicated film scanner, you must use a film based target, obviously. I’m personally not aware of any OEM scanning software that offers a calibration option. Both Silverfast and VueScan – two very good third party scanning software packages – have a calibration option and both work very well. Some of the other companies like ColorVision and XRite also have options for calibrating a scanner. Below is a screen print of the start of the calibration process.
This is a comparison of a straight scan before application of the scanner profile (left) and after (right).
The differences in the colours is pretty evident.
How do you use a scanner profile? Pretty simple, really. My preferred approach is to scan the film with no colour profile embedded. I feel this gives me the most flexibility. If I scan with a colour profile already embedded then I’m already starting to process colours. With no embedded profile, I’m getting as close as I can to a RAW image to start working with. Once the film is scanned, I use the Assign Profile option in Photoshop (that’s Assign, not Convert To). Assign profile actually alters the colours which is what we want in this case. Convert To just remaps the colours from one colour space to another but doesn’t alter the colours except for any out of gamut colours (more on that later). In this case we want the colours to change because we want the colours rendered by the scanner to be corrected for any ‘errors’ in the way the scanner reads colour. After I’ve assigned the scanner profile, I then use the Convert To option to bring the image into my working colour space. My working colour space is ProPhotoRGB. Why? More on that later.
That’s it. You’ve scanned an image, corrected the colours and have it ready to begin edting.
Calibrating a Monitor
NB – Some recommend not adjusting the RGB sliders to set the monitor colour temperature and that if your LCD monitor doesn’t have specific colour temperature adjustments, leave the RGB sliders at factory default and allow the graphics card/profile combination take care of the difference. I’ve done it both ways with equal results. The upside to not adjusting via RGB sliders is that the calibration process takes much less time; a benefit that can’t be overlooked.
Calibrating your monitor is another fairly simple process but it can take some time. I use the ColorVision Spyder2Pro and the screen captures below will be from the process with that device. Other monitor calibration systems will go through very similar steps but the actual screens will look a bit different.
The process takes about 10 to 15 minutes from start to finish. I won’t go through the entire process but will walk through some of the steps with screen captures to show what I’m talking about.
The screen shot below is the beginning of the process and it’s where you choose the parameters that will be used for calibration. You can see that I’m choosing a Gamma of 2.2 which is standard. A Gamma of 1.8 historically was common in the Mac environment but using 2.2 on a Mac may also be preferred if you’re going to be sharing photos with non-Mac users. If you’re on a Mac and everyone else you know is on a Mac and is calibrated to a 1.8 Gamma, then go ahead and use this.
Before going any further, it’s probably worthwhile going into what Gamma is. It’s go several meanings depending on what the reference is but as it relates to digital photography and monitors there are two points of importance. Once is that as it relates to monitors it’s a correction for the difference between the power input to a monitor and the brightness the monitor displays. That relationship is not linear. The other important point is that Gamma is a correction for the fact that human vision is nonlinear and that we perceive colours differently depending on the level of brightness or amount of light. A Gamma of 2.2 is considered to be close to human vision and more accurately represents how we see reflected colours in a print.
To continue on, you see that I have the option in the lower left to keep these monitor settings or change them. The values for Black Luminance and White Luminance are the values that were measured from a previous time I calibrated my monitor. These may change over time as the monitor ages so selecting Change These Settings to remeasure the values each time is advised. You’ll see that I’ve chosen a colour temperature setting of 6500K. There are debates in some circles about whether 5000K or 6500K is the proper setting to use. There’s really no “right” number. I use 6500K because I find at 5000K the monitor colours are too warm and a bit dull and I don’t get the consistency I’d like between my prints and what I see on screen. At 6500K, the colours are more vibrant and what I see on screen and what I see in a print are much more pleasing to my eye. If you try 6500K and don’t like it, don’t use it. If you try some other value and get a better result, use that. 6500K is also generally closer to the native colour temperature of most LCD monitors and the closer you can keep things to their native state, the better off you’ll be.
Next I choose what type of device I’m calibrating and what adjustment controls are available to me.
From here the calibration process begins. The colorimeter placed on the screen begins reading colour blocks and an adjustment screen comes up to tell you what current values are and what the target values are. You adjust the controls on your monitor and Update the readings until your adjusted settings are within the proper range.
From there, the device will read another series of colour blocks and a screen will appear to adjust the Brightness controls. Again, make the necessary adjustments and remeasure with the Update button until the actual value meets the target value.
Once complete, the device will read another series of colour blocks and create your ICC profile. You name the profile, select it as the default and it will be loaded into your graphics card each time you boot up your computer. If you want a more in depth look at display profiling, check out the Advanced Display Profiling sub-article.
How often should you go through this process? It depends. It depends on the type of monitor you have, how old it is and how much it gets used. As monitors age and are used, the colours that are rendered change. It’s called drift. CRT monitors drift more and sooner than LCD monitors. The more a monitor is used, the more and sooner it will drift. If you’re using a CRT, using it a lot and doing a lot of work where colour accuracy is very important, then once a week is probably a good starting point. Professional graphics shops will calibrate their displays every day. If the CRT is more than a couple yers old, more frequent calibration is advisable. With an LCD monitor, you can get away with doing it less often. Every couple of weeks or every month depending on how much you use it. I typically recalibrate mine every couple weeks.
Do you absolutely need a hardware calibration device? Nope. The process is easier and more accurate with one but you can do a very workable job with software alone. One very good (and free) software option is Monitor Calibration Wizard from Hex2bit software. I’ve used it and it can work very well. Calibrating a CRT with software alone is easier than an LCD. With an LCD you may have to go through a few iterations before you get it right. Personally I would not recommend Adobe Gamma. It comes with Photoshop and Photoshop Elements and really does a pretty poor job. A software only solution should really be a last resort. While it will be better than nothing, it’s not going to be nearly as good as a hardware solution.
One other thing to note is that if you have Adobe Gamma and Adobe Gamma Loader installed on your computer, you will need to disable these from your Startup options. If you don’t there is likely to be a conflict between Adobe Gamma and the profile you made and colours won’t display properly.
Once you’ve got your scanner (if you use one) and your monitor calibrated, the next step is to make sure you’re using good profiles for the papers you use in your printer. Due to the coatings and warm/cool tone of the papers we print on, without a good profile, the likelihood of getting a proper print is quite small. Fortunately, most quality paper manufacturers provide profiles for their papers with many different printers. These are called “canned” profiles because they’re generic for all printers of a certain model (e.g., Epson 2400, Canon iPF5000). Custom profiles are ones which are made specifically for your printer.
Canned profiles have come a long way in the last 5 years or so. At that time in the past they could be quite inconsistent and often times not overly accurate. Today, canned profiles are generally very good and will give you a very pleasing result. So much so that custom profiles really may not be necessary if canned profiles are available. If you want an absolute exact profile for your printer, to remove the result of any manufacturing tolerances, then custom profiles are the way to go. If you want to be completely anal retentive about it, you should redo the profile when you get a new batch of paper of a particular type or when you replace ink cartridges in you printer.
The way you make custom profiles is with a device called a spectrophotometer. I’m not going to go through the process in detail but it’s very similar to scanner and printer calibration. You print out a colour block chart and use the spectrophotometer to read the colour values. The colour values read are compared to the actual values and a set of differences is created which is the basis for the ICC profile. That ICC profile gets stored on your computer and is available for use when printing. The profile interacts with the printer to tell the printer how to lay down ink on the paper to accurately render colour (or shades of grey for a b&w print).
You can also create profiles with a scanner. Scanner based profiles aren’t typically overly accurate and I wouldn’t recommend that approach. There are also software only options for creating profiles. These are based on your visual accuity and adjustments you make manually to the settings in the software. You print a test page and adjust the settings in the software until you get a good match between what you see on screen and what appears on the paper. It’s a more iterative process, wastes ink and paper but you can get a decent match. It’s not as accurate as with a spectro but it would be better than using no profile at all.
How to Use Printer Profiles
There are a couple of places you can use profiles. One is in a process called Soft Proofing and the other is in the actual printing process.
Photoshop Elements doesn’t yet have a Soft Proofing capability built into it (v6) but the full version of Photoshop does.
Soft Proofing is a process to preview what a print will look like so you can make any desired changes before committing ink to paper. You activate Soft Proofing by going to View>Proof Setup>Custom. In the dialogue box that appears, you select the profile for the paper you’re going to print on from the dropdown menu. Often this won’t cause a big difference in the look of the two images. Typically there’ll be a bigger change with a matte paper than a glossy/lustre. To really see what’s going on, you need to mimic the paper you’re printing on and to do that, you need to check Simulate Paper Color in the Display Options (On Screen) box at the bottom of the proof dialogue. This is where you’ll see a bigger difference, particularly with matte papers. What this does is simulate the white level of the paper and it can have a significant impact on the contrast and saturation of the image. You can toggle Black Point Compensation on and off but probably want to leave it on. BPC works to properly map the black point of the image and other darker/shadow tones in the original image to better represent them in the destination (print). Many papers don’t have as dark a black point as our digital images or can’t produce some of the more subtle shadow tones which can result in a loss of shadow detail. BPC will remap the blacks in these cases to preserve shadow detail and render the deepest possible black for black toned areas. In very few cases, having BPC turned on won’t produce a good result and will work to wash out shadow/black areas. If this happens with your image/profile then leave BPC off. BPC also will have more impact when using Relative Colorimetric as the rendering intent than when using Perceptual. When you do this, you’ll see the colours in the image change. This is showing you a representation of how the colours will appear on paper. If you want to adjust the photo before printing to correct for any undesirable changes, you can save this Soft Proofed copy under a different name, reopen the original file and with the two side by side on your screen, make changes to the Soft Proofed version (on a separate layer is best) until it matches or more closely matches the original. The two screen shots below show the Soft Proof screen and a comparison of the two side by side. The other dropdown menu in the Soft Proof dialogue is the Rendering Intent. This determines how out of gamut colours are remapped. I’ll discuss Rendering Intents more below. For regular printing, either Relative Colorimetric or Perceptual will be used.
You may not like the look of your soft proofed image compared to the non-soft proofed original. There are things you can do about this. Turn off soft proofing and duplicate your image (Image>Duplicate). Tile the two so they’re side by side at the same zoom level (Window>Arrange>Tile Vertically). Now apply your soft proof to your original image. There you go, you’ve got the proofed and unproofed side by side so you can see exactly what’s happening. Now you can add additional adjustment layers; typically a Hue/Saturation layer and a Curves layer do the trick, and make adjustments as needed to get your soft proofed image as close as possible to the original. Name these new layers so you know specifically what they do. You can group the layers (select the print adjustment layers and go to Layer>Group Layers) and name the group to keep these layers separate from all your other adjustments. You can then save the file with the print adjustment layer group intact and turn the entire group on and off as needed. With these print adjustment layers in place, you can then tweak the adjustments as needed for soft proofing with other papers.
Going hand in hand with the Soft Proof capability is the Gamut Warning feature of Photoshop. This feature lets you know what colours in your photo can be reproduced in the print and what colours will be remapped according to the rendering intent.
Once you see what colours are out of gamut you’ve got a couple options. You can do nothing and just let the rendering intent take care of things. There’s nothing wrong with this. If; however, you want to take a little more control of the process, you can make some additional edits to the colours in your photo to manually bring them into gamut. These adjustments can be done via layers and grouped with your other paper specific adjustments.
The other place to use profiles is in the actual printing process. The screen shots that follow are from an Epson printer driver. Different drivers will look different but the same selections should still be available.
Once you’ve got your image soft proofed, any final tweaks made and it’s time to print, the printing process is the last step in the colour managed workflow. You can’t just hit the Print icon at the top of the screen. This too is an interactive process.
The steps outlined are the way I do it. Others may have different steps or go through the process a different way. The point really is that there are certain keys to using profiles properly in printing irrespective of the steps you use to get there. The steps shown below are for printing from Photoshop CS2. The steps with CS3 are slightly different and some of the screens look slightly different but the overall process is the same.
The first step is to go to File>Print with Preview (File>Print in CS3 – a summary of differences in CS3 is presented at the bottom). When you do that, the dialogue box below appears.
The selections here are as follows:
1. Color Management
2. Document (this shows the colour space the document is in)
3. Let Phtoshop Determine Colors – this is vitally important. You want to make sure Photoshop is managing the colour and not your printer driver. This is the only way the proper profile will be used and the only way to have a properly colour managed workflow.
4. Printer Profile – this should be the profile for the paper you’re using and it should be the same profile you used in the Soft Proof step.
5. Rendering Intent – You’re going to use either Perceptual or Relative Colorimetric, whichever you settled on when you soft proofed. Ignore Saturation and Absolute Colorimetric. I’ll discuss what each of these means below and what impact they have on a photo.
6. Black Point Compensation – should be on or off to match what was used in the soft proof.
Next I click on Page Setup and choose my printer from the dropdown menu. From here I go into Page to select my paper size and orientation then on Main to select my paper type (e.g., Lustre, Watercolor), then select Color or Black and White then on Custom and Advanced.
The settings in the Advanced section are very important. At the top is the resolution to be used. I generally use 1440 on matte surface papers and will sometimes switch to 2880 on high gloss papers or unless a specific profile I’m using has been made for the 2880 setting. Below that, none of the boxes should be checked. Typically High Speed is turned on by default, make sure it’s unchecked. On the right side of the screen is the most important setting and this is where you want to ensure that Printer Color Management is turned OFF. If you don’t do this, all of the work you’ve done up to now will be for naught because the printer driver will take over and your paper profile will be useless.
From here, click OK all the way back to the first dialogue box then click Print to send the job to the printer. That’s it, you’re done.
One of the things people often say when printing is that they’re looking for a ‘match’ between what appears in the print and what they see on screen. I’m sorry to tell you but that’s never going to happen. You’re never going to get an exact match. A big reason for this is the two different light sources used. Viewing an image on a computer screen is using transmitted light. It’s almost like viewing a slide on a light table. The light is shining through the image out to you. Prints, on the other hand, are viewed with reflected light. Light hits the paper, some is absorbed by the ink and paper and some is reflected back to our eyes. The amount that gets reflected back and how it gets reflected back determine brightness and colour. There will always be a difference between transmitted and reflected light. Images viewed with transmitted light will always appear a bit more bright and vibrant. So you’re not looking for a ‘match’ between the two. What you’re looking for is consistency of colour and a print that is pleasing to the eye. That said, there shouldn’t be black spots in the print where you see detail on screen. If there are dramatic brightness differences, then there’s likely a problem with the paper profile or with your monitor calibration.
In some cases you may find that there are profiles available for the same paper/ink combination under different lighting conditions. If you’re having a custom profile made or making your own, a choice of ambient lighting may be offered to you by either the software you’re using or the service bureau that’s making the profile. Unless you know with certainty what lighting condition your print is going to be viewed in and that it will always be in that lighting condition, stick with the default profile condition which is a daylight balance. Profiles are created for different lighting conditions to take account of metamerism inherent in digital prints. Metamerism is the shift in hue that occurs in colours when viewing them in one light source vs. another. It’s expected. The biggest reason I don’t believe profiles for different lighting conditions are necessary is because human vision has a terrific adaptability. Human vision has a characteristic called colour constancy. Colour constancy is the idea that we see colours as being the same irrespective of the colour temperature of the ambient light. While colour constancy in human vision isn’t perfect – it gradually fails as the light colour temperature becomes increasingly skewed to one end of the spectrum or the other, even in extreme lighting, it still has an effect. The net result is our eye/brain combination pick up on a colour in the print; particularly white or memory colours (red, green, blue, skin tones), recognises that colour and then automatically adjusts what we see so that the colours in the print remain very accurate. As photographers, we’ve trained our eyes to see light and as a result we readily see the cool light of shade or the warm light of a sunrise but our brains are still making adjustments even in those cases so, in my opinion, there isn’t a compelling need to have profiles for different lighting conditions.
What is a Rendering Intent? The rendering intent is the way in which out of gamut colours are managed. Out of gamut colours are those that can be reproduced in one place but not in another. Each colour space or printing paper has a defined set of colours that can be reproduced. When you move from a larger colour space to a smaller one there are likely to be colours that the smaller colour space can’t reproduce properly. The rendering intent determines what happens to those colours. There are four choices.
Saturation – mapes all the out of gamut colours to the closest in gamut colour. It also remaps some of the in gamut colours closer to the limit. What can happen is that you can have a marked increase in saturation of some colours that can look quite unnatural. Saturation also doesn’t necessarily map to the same colour family. If you’ve got a highly saturated red that can’t be reproduced in the smaller colour space but a green with the same saturation level can be represented, the red may get mapped to the green. Not good.
Colorimetric, Relative & Absolute – both handle out of gamut colours the same way. All out of gamut colours are remapped to the closest in gamut colour. The result can be an increase in saturation of some colours. But colours are still mapped within the same colour family. Where the two differ is in the way they handle the white point. With Relative Colorimetric, the white point of the source image is remapped to compensate for differences in the white point of the destination; white stays white (as much as possible). Absolute Colorimetric doesn’t compensate for the white point differences. Absolute Colorimetric is good if you want to replicate printing on one type of paper by printing on another type of paper. Absolute Colorimetric will add the colour of the paper you’re trying to replicate onto the paper you’re printing on. Relative Colorimetric won’t do this.
Perceptual – remaps all of the colours; both in gamut and out of gamut, so the relationships between the colour values are maintained. It will sometimes result in a slight drop in overall saturation but because all of the colours are remapped, the perception of the saturation drop will be less. This is a good starting point to use for printing and soft proofing. If you don’t like the result, switch to Relative Colorimetric and see if you like it better.
Based on what each of these rendering intents does, it becomes fairly easy to see why Perceptual is a good choice. Toggle between Relative Colorimetric and Perceptual to see which works best for the particular image when you’re soft proofing.
The last piece of the puzzle in the rendering intent puzzle is Black Point Compensation. Turn it on or leave it off? It really depends on the image as it may differ from photo to photo. What it does though is remap darker tones to compensate for the ability of the paper to render darker tones. With BPC turned off, you may lose some shadow detail because darker tones past a certain point may all get rendered as black because the paper can’t reproduce the very subtle differences in tonality. With BPC turned on, darker tones are remapped (lightened slightly) compared to the darkest tone in the image to try and maintain the tonal relationships as closely as possible. If you’ve got subtle shadow detail you want to try to maintain, having BPC turned on is probably your best bet.
Lastly let’s talk about the colour space you use in your editing. You’ve got a number of choices: AdobeRGB 1998, sRGB, ProPhotoRGB, Wide Gamut RGB and so on.
My personal philosophy is that more is better. Using a wider colour space as my working space and tagging my images with that colour space gives me more flexibility. I use ProPhotoRGB as my working space. This is a very wide colour space. Much wider than AdobeRGB and vastly wider than sRGB (AdobeRGB is wider than sRGB). Monitor technology is changing as is printer technology and there are printers on the market that can render more than AdobeRGB in some colours. If you start out with an image in the sRGB colour space you can never convert it to increase the breadth of colour and take advantage of new technology. If you start out with a wider colour space, you can always convert down if needed.
As mentioned, there are some subtle differences between CS2 and CS3. In CS3 it’s simply File>Print rather than File>Print with Preview. The updated GUI is quite nice.
Adobe has effectively built the soft proof feature into the print dialogue in CS3. The checkbox called Match Print Colors in the lower red circled area will soft proof the image for you based on the profile and rendering intent you choose. What it does is basically streamline the Print with Preview path from the previous version and give you an onscreen preview in real time. Does this mean using Soft Proof is no longer needed? Not necessarily. If you don’t like what the proof looks like here you still need to go back and tweak it in PS. It also doesn’t give you a gamut warning, you still need to be in PS for that. It’s just a nice feature to have built in and it’s nice that Adobe haver streamlined the process.
Another change in CS3 is in the red circled area on the right. If you select Photoshop Manages Colors from the Color Handling dropdown menu, you’ll get the reminder message below to confirm you’ve turned off colour management in the printer driver. Again, it’s a nice feature that may help ensure the print turns out properly.
Other than that, the printing process in CS3 is much the same and it’s exactly the same in the printer driver itself.
A Final Note – Prepping for Web Display
The concept of colour management for presentation of photos on the web is a bit different. Most web browsers aren’t colour managed. That is, the browsers don’t have the capability to adjust colour for different profiles. That doesn’t mean we can ignore colour management and colour profiles altogether. When posting on the web, in order to maintain colour fidelity and contrast you want to convert your image files to the sRGB colour space. You should only do this for images that are specifically being saved for posting on the web or sending via email. That is, only convert to sRGB when you’re ready to save a small JPEG of the image; don’t convert your working TIFF to sRGB. If you post in AdobeRGB the colours will look flat, washed out, perhaps a bit darker and you’ll often lose contrast. Many times people will post a photo that isn’t tagged with a colour space at all and this too will result in flat colours and a lack of contrast.
The photo on the left below has been left in my working colour space of ProPhotoRGB. The one on the right has been converted to sRGB. The colours in the lily on the right are accurate whereas the colours in the one on the left are washed out and dull.
And remember, you want to use Convert not Assign for this.
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