Thanks Moscatel.
Actually, I learned more about colors when I worked in watercolor. Digital tends to use the colors already mixed. I do some mixing on the "canvas" but not as much as with traditional painting. The one thing I do make sure of color wise is to make sure colors vary slightly in a large area of the same color. If I just use the same color setting, it looks very flat and there is no life or spark there. You only have to vary the color slightly but it's important to do so.
You are right on about composition though. I work in layers, so I can resize parts of the composition and move them around. I can make the whole thing large enough to fill the canvas even if I didn't start out that way or shrink it down a little if I worked too large to fit (a common mistake I make!).