Before you start shading, always figure where your light source is hitting your painting. This can take quite a while to learn. Sometimes I get this aspect wrong. First, start off the shading with natural mixtures of colors to the skin tone. After you’ve done that, add some darker hues to the skin. Save some space at the edges of the face for your back-lighting. If you don’t know what back-lighting is, it’s the reflective light that shapes your face in certain types of lighting.