#1 Nail down the lighting.
(Sources, direction, strength. etc.)
#2 Nail down the value relations.
For instance, the tree behind the dwarves: is it more attractive as darker than the background or lighter? By exploring and solving this in the color comp I will be more confident when I tackle it in the final image.
#3 Nail down the color theory.
This image will be confined to a warmer spectrum, one that you might find at midday in an old growth forest with patches of sunlight breaking through the canopy. I wanted to make most of the tones fall in a rather narrow color gamut, and then choose a bright color outside this gamut as an accent. In this case, it was mostly golds and browns accented with a bright green.
In the color comp I just want to make sure this theme will work for my image. I go through this on almost all of my pieces because my brain is incapable of solving value/color mixtures on the fly, so I need a cheat sheet. The color comp helps me idiot-proof my image so I don't bungle it in the final.