I've been trying to figure out a solid way to generate custom color separations of an image. Sort of like taking an RGB image and separating it out into CMYK only instead of Cyan or magenta I want to get, say, an orangish-brown channel or a bluish-grey channel.
Think of it like separating an RGB image into spot colors for screenprinting. If there's an orange-brown color in the image, I'd be separating out a screen just for that color. I'm taking RGB images of natural stone and separating out the natural stones colors for screenprinting. It's usually a four-screen process with one screen being, for example, a light-beige, the second screen a maroon shade, the third a greyish color, and the fourth a dark brown.
There was a plug-in made by a company in Spain called inedit Multicomia, but it never got out of beta and never really did anything without crashing.