If you are trying to be realistic, there are many color variations within the face, and all need to be custom-made per character. If you are looking for just basic color that matches the bodies and are going to leave it reasonably flat in color hues, there are a few decent "pre-made" skin colors. But it does not really match the Hot Toys' bodies out of the bottle. So even those would need to be tweaked and hand mixed.
Sorry, I know it's not the answer you were probably looking for, but hopefully, it helps how you attack it. Good luck!