I faced the same difficulties when trying to adjust the color of a poser5 face to the skin of the body. But I adjusted the darker skin only in Photoshop to the level, I needed. Another way is to adjust the lighting of the darker face from poser 5 by changing texture in the face room to “more European”. Could be same in DAZ Studio.
The poser chracters are using two different texture maps. One for the face and the scond for the body.