If you make one layer, say the skin, and you want to shade it, make another layer directly above the first, and press control+g or go to Layer then Group with Previous. That way, when you shade on the new, grouped layer, you can only color where the initial layer covers. Basically, you can't go outside the lines (lines in this instance being whatever you colored with the "skin" layer) and you won't have to do the step of erasing the shading.
I don't recall. Is this something I'm going to have to just resize to get it to not be pixel-y? XD