my biggest tip: mess around with different combinations of hue, saturation, and lightness in your colors, to create what's called a color ramp, a series of colors which conveys differently "shaded" parts (your highlights, your midtones, your shadows). keep in mind, as areas get darker, they tend to not only become less light, but more blue/green, too.
also, don't be afraid of high contrast between your different colors. a lot of your colors blend in (you have 4 reds, i can barely make out 2; you have 4 grays, i can again make out 2; etc). zoomed out, i can't tell them apart, and so really they don't add anything! contrasts between your colors flesh out a piece, and give it a lot more depth, since the eye easily picks up "ok, this part has a deep shadow, it must be ..." and then fills in the blank.
if you have trouble, report back. i think that you seem to have a pretty good grasp on shape, and so picking up color shouldn't be too hard, but better to experiment first so we can help you more in depth
