The problem with the melanin hue adjustment is areas around the hand palms and foot soles are where the skin is entirely different in those regions making them lighter in color.  Either you need to apply a mask or define those areas as separate material zones.  This reminds me of the old Sci-fi TV show 'The Invaders' created by Larry Cohen where the aliens could not replicate human skin precisely because the hand palms retained the same coloration as the rest of the body.