Dr. J. Smith wrote:
Why is it if I desaturate an image in RGB then convert it to CMYK and desaturate it again it’s a different shade?
This turns out to be an interesting question, so the answer will be longer than the question by quite a bit 🙂
Desaturate uses mathmatics based on the HSL color space. HSL is defined geometrically in terms of RGB ratios.
For Hue/Sat/Lightness calculations, Photoshop treats a CMYK image as an RGB image, ignoring the K value. By this logic, when a CMYK image is desaturated, gray is generated by creating equal amounts of CMY inks. Since cyan is the weaker of the three inks, a desaturated CMYK image will take on a red cast.
One servicable work-around is to convert to an RGB working space for the desaturate operation, then back to CMYK afterward.
This quirk also affects the layer modes, the sponge tool, Image>Adjust>Variations, and Image>Adjust>Hue/Sat . For example a CMYK layer set to luminance mode will impart a red cast to the underlying layers. I recall reading that some of these operations have been changed in Photoshop CS, perhaps others will comment on this.
—
Mike Russell
http://www.curvemeister.com http://www.zocalo.net/~mgr http://geigy.2y.net