Painting the Adult Human Head

Build up thin layers of skin tone, being sure to avoid all areas of highlight. Introduce planes where you see them and be sure to look for shapes of shadow and treat them like structures. Burnt Sienna washes can be a good start for a fair skin face, with yellow ochre a good starting point for those who have a warmer complexion. After the base wash dries, add light glazes of alizarin crimson to call out blood vessels close to the surface. Burnt sienna and yellow ochre can be glazed as needed.

Some of these strategies are discussed at this Artist Network article.