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I have some images to send to the printer. The printing company wants them in 'CMYK' but doesnt offer any other information. I've read up quite a bit on this but most info that I've found seems to be about the theory of color profiles with not much on the actual procedure.

Currently my image is in sRGB (I'm using the GIMP which I believe uses sRGB, but doesnt embed this color profile in the image). I understand that ISOcoated_v2_300_eci.ic is a good general-purpose CMYK profile.

1) Do I want to convert my image to the CMYK profile or do I just want to add the profile to the image?

2) How can I verify that the above has worked. E.g. if I load the image up with some viewer app and view it on the monitor, how can I tell if it is all correct? I.e. I believe once the image is in the new profile you should be able to preview to get an estimate what it will look when printed (as CMYK has a smaller range than sRGB) and maybe adjust the colors to improve things. Maybe there are some test images showing before and after conversion?

3) Can anyone recommend tools for 1 and 2 (free hopefully). I am on windows 7, but could also use Linux. (Other than Separate+ for the GIMP - I tried it but I'm pretty certain its not working correctly for me as yellow turns into yellow-green).

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1 Answer 1

GIMP doesn't do color profile conversion very well AFAICT. Based on what I've read in the past, it still doesn't have full CMYK support. I personally use Photoshop, and for simple color conversion, you can probably just use an old copy of Photoshop (even as old as Photoshop 6.0 if it'll run) that you can pick up pretty cheaply.

You almost always want to convert your colors (Edit>Convert to Profile...) when changing color profiles (unless you mistakenly tagged/assigned the image with the wrong color profile before or received an image with no color profile attached). When you convert to a different profile, there will still be a visible color change, but it will be minimized based on the conversion method you choose (Perceptual, Saturation, Relative Colorimetric, Absolute Colorimetric).

If you're starting with an RGB image, there's no way you can assign a CMYK color profile to it, but Photoshop will still let you change the color mode without using the Convert to Profile... command. Photoshop is still going to do some form of color conversion behind the scenes, but I'm not sure which, and in my experience, the printed results are usually color shifted far more than when you use the Convert to Profile... command.

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@spiderplant0: You can't tag an RGB image with a CMYK color profile because RGB has 3 channels while CMYK has 4. Therefore, some form of color conversion needs to take place. You can assign a different RGB profile to an RGB image without color conversion because it's still going to be 3 channels at whatever bit-depth the image is. Image viewers will simply display different colors for each RGB tuple. –  Lèse majesté Jun 30 '13 at 20:48
    
See en.wikipedia.org/wiki/Color_gamut and en.wikipedia.org/wiki/Color_space to understand why changing color profiles can change the colors of an image. –  Lèse majesté Jun 30 '13 at 20:50
    
Thanks Lese, understand now. –  spiderplant0 Jun 30 '13 at 21:33

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