It occurred to me that using a Hue/Saturation node rather than a color mix would give more control, as well as more realistic colour for skin texture.
So I tried it and am, so far, very happy with the results. Here are some examples:
1) Default skin tone
2) Tanned
3) Sunburnt
4) Hulk Look
The inputs on the Group node correspond to those on the Hue/Saturation node with the exception of hue, which is split into Coarse, Medium and Fine adjustments:
5) Screenshot
If anyone would like to try this, I've attached a .blend file with nodes set up for body, lips, ears and nipples.