I use both bottled Testors colors and oils to paint figures. For smaller scale figures (120mm and smaller), I've been using the new Testors flesh system. It's about six bottles of different shaded browns and tans that, when used logically, look great. For larger figures I use oils so that I can take my time painting.

My basic flesh color is mixed up by using a base color of Gold Ochre, adding white, burnt sienna, raw umber, and brilliant red as needed. Make sure that you use Gold Ochre, not yellow ochre. The yellow ochre turns a sickly color after all of the shading colors are mixed in. As for how I mix it, I just go on instinct. Basically I mix up some paint, match it to my skin, my son's, then my wife's, outside. If it looks to be within the spectrum of our skins, I use it. So far I've been, IMHO, successful in catching the right tones.

Skin is hard to paint, especially convincing African skin. I did a Zulu Warrior once, he looked alien! Keep practicing and repainting, eventually you'll get it. A good source book to help you get a leg up is Shep Paine's Building and Painting Scale Figures, it's my New Testament of modeling books!