I typically paint from scratch. I think overpaints wouldn't really help me learn anything, I've got to actually start things from the ground up. Plus that's more fun and interesting for me to do. Maybe I'm not understanding though on that shadow bit. My monitor that I work off of, renders my darks fairly dark, but even still that first color speedpaint I did of the woman I put some lighter accents in her hair to break up the black. Granted it's not everywhere in the hair, just the top and near her face.
Thanks! I noticed that too looking back on those sketches. Yet another thing to try and be careful of. hehe