do you work with layers? if yes, then just add mask on that red part, put face layer under red layer and black layer with back part of head under it too, and then just edit mask of red layer with rubber, or try it with black layer (blank at beggining u will draw with black here) over red and with black brush make those strands of hair on back part of head, or with any other colour, its just on you, and this is just advice how i see it...