Skin tones in general and the painting of a model's face specifically is one of the key points to successful miniature painting. The following is a basic description of my approach to human skin tones. First, here are some examples of different variations in the shades and highlights on faces:
Same starting point - various finishing options
In 9 of 10 times I start painting skin with a base colour of Dwarf Flesh with a drop of dark green ink. The addition of the green ink serves to break the otherwise very pink colour and changes the colour in a greyish direction depending on the amount of green ink added.
When choosing the shading colour it is decided in which way the further painting of the skin will go. For most human skin types I use a thinned mix of Chestnut and Purple ink. The relationship between Chestnut and Purple (and water) varies according to how deep I want the shading to be. Sometimes I find it necessary to apply 2 layers of shading to get the shade I want.
Other options can be to go pure (plus the obvious water for thinning) Chestnut (the mutie leader above is an obvious example), Purple, Brown, or maybe Chestnut plus for instance Snot Green for a colder greyish shade.
The highlighting process starts by touching up the base colour and then adding white to the base colour for further highlights. I usually do 3 to 4 succesive highlights where the final highlight is particular important and needs to be painted with a good contrast.
Instead of adding white to the base colour another approach is to use Elf Flesh after the Dwarf Flesh base colour and finally use Elf Flesh plus white for the final highlight.
Sometimes glazing an already finished area can add some extra dept to the colour. For human skin I like using a very thin green glaze (above on the Mercenary captain and around the eye of the Keeper of Secrets) to contrast the warm shading in the face. It also works the other way around where for instance green orc skin can be glazed using red.
Having glazed an area I usually go back and redo the final highlights to make the features stand out.