For example, for a simple cauacasian flesh tone, you could choose either Beige Brown or Brown Sand as your base shade, then follow that up with either Beige Red or Med. Flesh, and finally a highlight of Sunny or Basic Skintone. Or you could start with a base of Beige Red followed by Flat Flesh and Basic Skintone.
The point being that these colors work with one another decently, and by varying your triads, you can vary the composition of your army's faces.
I've also put together a basic African and Indian palette. I kept these palettes simple because I think they are easier to put together. Just about any brown will work nicely. What makes caucasian difficult to recreate is that it's such a fussy combination of red, yellow, white and a tiny bit of blue without making it too red or too orange or gray. The you have to account for tanning skin or sun burn.
This is a very muted palette, lacking much with any red in it. This is why I added the Beige Red to the palette; it provides just enough blood under the skin to give the miniatures a bit of life. You could also, of course, mix any of these colors, light or dark, together to make an endless list of colors. But if you don't like to mix (and I don't), try out these colors.
Notes:-- My scanner didn't capture the colors exactly, but I listed the name and number of the swatch next to the color. The Sunny and Basic Skintone swatches, for example, are lighter and less "yellow" than they appear above.
-- Flat Earth actually has a bit of green in it. I think it actually works, and, as a base, also serves to gently compliment Red Leather and Light Brown. Experiment on your own; Vallejo has a lot of browns that work well.
-- The Basic and Sunny Skintones are much lighter in person -- the Basic being almost white when painted on. So use these sparingly, mostly as simple glints of light reflected off of skin (tip of the nose, a thin stripe on the top of each cheek, maybe over the eye brows.)
-- Asian skin is actually very similar to caucasian. But but if you want some differentiation, use either Beige Brown, Brown Sand or Medium Flesh as your base, staying away from the Beige Red (or any other reddish tones.)