If you put a piece of cloth on your face, the parts of the face that touch the cloth are the ones where color would be transferred from your face to the cloth, and which are therefore darker. The parts of the face that are more recessed, like the areas around the eyeballs, would not touch the cloth, and so less or no color would be transferred from the face to the cloth in those areas.

In other words, the part of the face that would receive more shadow in a normal image (and would be darker) would receive less pigment in the painting (and would be lighter).