This was done with a unique process a teacher taught me once. You lay down an acrylic base paint (light brown) and wait for it to dry. Afterwards you paint over it entirely with a gauche based paint (it's a water based paint and you want to use a dark brown). Then you will "lift" up the gauche with a wet paint brush. Basically "painting" the highlights by lifting up the dark paint. It's a lot of fun!
There is another step to this process after these two if you want to get more of a realistic skin tone. If interested let me know and I will try to explain. I didn't use that third step however on this one.