First thing I do is what we call "flat" an image. Basically, I select each object and fill it with the color I am going to use for it. It is at this stage that I establish the palette for the piece. I also begin to set the value relationships within the objects.
Someone asked me how I come up with color schemes for covers and pages. It's tough for me to answer that question because it just kind of comes to me. I know, sounds corny and stereotypical, but it just does.
Sometimes I will do a quick color sketch of it, especially if I am going to try something new or to work out any issues.