this diagram is great in showing how to shade in a sphere creating a realistic shadow and highlight. helps in understanding where the light and dark areas should be. using an aray of shades creates a more realistic and defined drawing
A Realistic Lip Painting Tutorial by Packwood on Deviant Art packwood. *NOTE, not my art- simply linking so other aspiring artists may use the tutorial for learning. I own no rights. You can use the included link to go to the original page.