Hi! I think you got pretty good base colors. Dark skin contains more melanin which affects how light is reflected back by the blood under the skin. The colors would change hue accordingly to more orange and violet up to grey/black depending on how dark the skin is (vs pink/red and blue/green like in case of light skin). Since the environment seems to be warm, I would use dark red/brown tones for shadows with some variation of violet (ofc it is not violet but a tone between dark orange and purple, see color mix). Same for the hair: a bit of dark beige and violet. Try to use dark red instead of black for lines. As far as shadow form goes: you need to think in 3D. It is worth investing into learning basic construction of the head as well as face planes. Since it takes time, for this specific drawing I would suggest using a 3d model of a head with similar light (or take a photo of yourself) as a reference (there are many apps that offer such function). I also noticed that glasses seem to steal the focus from the drawing. I would make them overall darker. Be it plastic or metal, they would reflect a bit of surrounding (orange tone of the skin). Side note: don't forget tear ducts! Hope it helps!