Found this in 2 sec (Google is your friend)
Think Base Color, Highlights and shadows. Basically you want to enhance the shapes thats already in the face sculpt. Some ppl also use a black base paint as its supposed to make it easier to modulate the Highlights and shadows later.
Start with a "Flesh" base color and look what the shadows and highlights do on the face, then simply start darkening the "Hollow/Negative surfaces" and highlight "ridges and peaks".
Normally I would use the "flesh" color as a base and then adding browns grays and blacks for the shadow and white, beige and lighter reds for the highlights.
My Medium for Face painting was always Oil colors as you can blend them beautifully. I tried with Acrylics but I never reached the same level of fluidity blending the colors, but I am sure there are guys that thinks differently.
Working with oil needs some discipline but as you need to be sure that the layers dry between each layer...about 24h or so.
But as already said there are tons of Tuts on line, face painting is a sort of science in it self if you want to do it hard core.