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Is it possible to convert an RGB image to CMYK without an ICC profile?

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AFAIK CMYK is always device specific. That would mean that CMYK without a color profile is not very meaningful. – bhell Dec 11 '12 at 21:50
Proofing CMYK in the RGB color space always requires a profile, yes, and is probably the preferred way to work. That said, more information is needed to give a definitive answer. – Dan Wolfgang Dec 12 '12 at 0:25
I'm pretty sure that Photoshop introduced the ability to convert to CMYK (1991?) before ICC Profile started to become a standard (2003?). But even then there would have been some kind of color space conversion data ("profile") to make the RGB Data -> CMYK Data -> Monitor RGB path work. It just wouldn't have been called an ICC Profile, that mechanism would be no more than a historical curiosity at this point. Photoshop still allows you to kind of mix your own ink colors as part of a custom CMYK profile, but I'm guessing that even that the process has an ICC foundation now. – David Rouse Dec 12 '12 at 16:25

1 Answer

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Short answer: If you don't mind which colors the CMYK channel values represent, simply apply any random CMYK profile you can find and call it a day.

Long answer: there is no really color space called "CMYK" with the meaning that ANY value combination for channels C, M, Y and K results in any specific color. Sure, a high value for Y and zero for the other channels should output some kind of yellow. It could be reddish or greenish, dark or very light, you cannot know. It all depends on the ink and media used for output. The same can be said about "RGB" colors but usually you can take it granted that "RGB" really means sRGB which is a real color space with a strict specification. An ICC profile for CMYK output device describes the end result of ink and media combination for any value combination for channels C, M, Y and K. If the intended result is to get similar color from the CMYK device as RGB device, you need to use correct ICC profiles for both. Most of the time, the end result will be really close even if you just apply official sRGB profile to the source.

If you don't know the profile of the output device, doing "color separation" (converting from sRGB to unspecified/random CMYK color space) is pretty pointless, in my opinion.

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