For a number of years, I used oils, mixing burnt sienna and titanium white in varying degrees for caucasian skin.
I've transitoned to Andrea/Vallejo acrylics. My base flesh color in red beige. Using the method suggested by the manufacturers to get good blending, I use beige brown and beige brown/beige red mixes for shadows. Foe highlights I light the base with light flesh and end up with straight light flesh for highlights.
For Native Americans, I darken the base with beige brown to start and might use a reddish brown filter to shift the color. Other shadows and highlights remain the same as above.
For a generic mid range African skin, burnr umber is a good base, very dark brown shadowsand raw umber for highlights.
For eyes, I use Vallejo light flesh, never pure white, with a drop of light or dark blue, brown or black for the iris.
See www.acrylicosvallejo.com, choose "Model color" from the drop down menu and scroll to the bottom of the page for their tutorial.