Apply shadows to the skin
Shadow edges and fill
When light strikes skin, it bounces beneath the surface and between the planes of the body. I use an Overlay layer and saturate edges between light and shadow areas to indicate where the light bounces beneath the skin. I also indicate light reflected from facial planes, such as under her nose. Experiment with the value of the Overlay colour so it doesn’t alter your skin values.
I give skin colours extra attention. While the blue of the underpainting will provide a lot of the shadow colour for the background, I paint the skin shadows more carefully. I want to cool them down, so I use an Overlay layer with a desaturated blue. I often try several colours before finding the right one. I add this to the shadows with a soft brush, avoiding the light edge.