I mix all my skin tones individually, it helps avoid them looking too uniform and lets me get the exact tone I want. Ultimately, it's mostly the same 4-5 paints that get used.
Vallejo Model Air Ivory
VMA Pale Blue Grey
VMA Dark Earth
VMC Royal Purple.
GW Mephiston Red (any strong red will do)
GW Agrax Earthshade
Always start with Red, Earth and Ivory, with more ivory for paler tones, more red for ruddier ones, more brown for a tanned or weathered look. Apply this all over (the mix should be pretty dark, to give good coverage and provide depth to the finished look). Then highlight up by adding in either ivory or pale grey depending on how vibrant or pale you want to go. Keep your final highlights nice and fine.
Then apply some (very thin) washes to add some colour to it. Watered down red will give you a warm look, purple a gaunt and cold tone and Agrax/brown a grimy look. You can also experiment with other colours here, green would give a sickly look, blue a chilled one. The key with these is to keep them really thin, so they don't pool and just tint the desired area.
A few examples, I'm afraid none of the pics are close-ups but you'll get the idea)
Hope that helps!