Usually I just do simple faces when I am forced to paint them. Tallarian Flesh, Tanned Flesh, Dwarf Fresh, Ogryn Flesh wash, touch up with Dwarf Flesh. The flesh wash has some red tones in it but I do not typically use any other colors. In almost any advanced flesh painting tutorial, you'll see people mixing in purples, greens and blues into their fleshes.
I touched my toe into this pool by adding some color to the lips with a Mechrite Red base and a light wash of very thin red and some purple paint and wash on the eyes.
I am mostly satisfied with the results. I want to experiment more with this and the Desert Wolves army is a good chance to do that as many of them will be missing helmets.