So that's dealing with CMYK to RGB? How do you do it in a proprietary software? I mean what is your goal exactly? Is it to have a printed picture, and to use it to make the sRGB version look as close as possible (provided your screen is rather neutral, and calibrated)?
I think GIMP has "some" CMYK support. And there are ways to colorize parts of an image. Inkscape has some CMYK support too, but that's for printing.
