The problem with painting flesh colours is that the shadows often look dirty and not too convincing. Restricting ourselves to just white flesh in this instance, because dark flesh is a completely different range of colours, here are some ideas. The colours I use are: Titanium White, yellow ochre, light red, indian red, Alizarin crimson, cadmium red, cadmium yellow light and cobalt blue.
Generally speaking I mix a standard flesh tint in the following proportions: 5 parts white, 1 part yellow, half a part of red; the mixtures are approximate. A cool flesh tone I would use Titanium white, yellow ochre and Indian Red, a warm flesh tint Titanium White, Cadmium Yellow Light and Light Red. Variations on this theme could be to replace Indian red with Alizarin Crimson in a cool tint and light red with cadmium red for a warm tint. The highlights can be a mixture of alizarin crimson and titanium white for cool areas and cadmium red and titanium white for warm areas.
Shadows, if they are very deep, could be a mixture of Indian red and Cobalt blue, this creates a cool mauve colour which is ideal.
Don’t be afraid to use blues, purples and mauves in shaded areas. As a very simple rule the areas of the face that are in shadow or further away paint in cool tones, the areas closer to you in warm tones. Obviously this will vary depending on light, angles etc. Lines on the face can be defined with Indian Red but this may well need toning down depending on the general colour scheme chosen.
Try to avoid mixing flesh colours with blue because they will be come muddy very quickly, also avoid the earthy brown colours (burnt umber for example) as they too become muddy when mixed with flesh tints.