here's an article that should help:
http://www.themonsterlab.com/vance-skinpainting.html
a more simplified method is this:
Use a base coat of a beige/flesh mix. Basically, a very pale, slightly peachy beige.
Use that new color as the base for all your other colors.
Mix a reddish brown with your base for a pinkish flesh that's darker than your base color. Using a torn house sponge, stipple on this layer over the whole thing.
Next, use a red-brown through an airbrush to shade a few areads (around the eyes, ears and lips as those are usually more "red" than the rest of the face.) Also hit the areas surrounding the wounds but not the very edge, you'll want that light to look "sickly".
Next, use a take your base color and mix in purple and gray. Spray through an airbrush to shade the eye sockets and lips.
Mix more purple into that color and add some bruising to the wound areas. This should mix well with the red-brown layers below.
Don't forget to mix some black in with your blood tones at the edges. It's a simple trick for a nice "gory" look.
Here's one i painted up some time ago for a member here. Unfotunately, this was before i got into the sponge methods and looks a little flat to my eye. Had i used the method above it would've looked much more alive! (BTW- the eye is a simple craft store faux jewel)
http://homepage.mac.com/bluesnal/.Pictures/Portfolio%20JPEGS/Terminator%20paint%20up.jpg
Here's some pics i took during the painting of one of my Peter Cushing busts. It's a pretty good example of the above mentioned method.
http://homepage.mac.com/bluesnal/tarkin/PhotoAlbum12.html
Anyway, the colors i'd get are:
largest amounts:
PALE FLESH and LIGHT BEIGE,
next largest:
RED-BROWN,
accents:
PURPLE (small amount), GRAY (small amount)
you can also try FREAK FLEX paints by Badger... lots of unsexy colors there!
hope that helps!
tom