For skin, I think variation of colors is important, otherwise you can end up with skin that looks flat and plastic. By that that I mean not only changing the values, but also playing with the hue and saturation a bit more when the skin goes from light to shadows. You do that already a little bit, but I think it could be pushed more.
Remember that different paintings will have different lighting, so this specific example of skintones will not work for all paintings, but it will hopefully show you what I mean