If you want to turn a caucasian model african american, then pick colors that match best how light/dark you want her to be. Keep in mind that skin has different colors in it, all blended to make on color, so if you do that as well when coloring, you can't go wrong. For example, a caucasian, very light skinned girl will have some pink in her skin (soft lighted so she doesnt look like a pig!) as well as a whitish color picked from the "yellows" range. Caramel colors will call for light yellows and tan or mocha colors, and dark skinned girls will have very dark brown, a small hint of yellow and even a very light shade of pink, burned a bit.

See her skin? Its very dark but the highlights in her skin show a bit of yellow.
Oh, and don't mind the horrendous hair lines, I never blended lol