If you want me to colour something, I require the picture, colour reference for everything in the picture (right down to the smallest piece of jewelery), and permission from the artist. Bribes don't hurt either.
I only do flats, not shades. I only colour stuff I like, as well. What I don't like is pretty much what most people don't like, that being scat, gore, etc. I'm fine with vore, though. Not my fetish of choice, but I can stand it.