Many thanks for all the nice comments guys - it's really motivating.
For the faces and skin I tend to use three colours. A flesh/brown, a middle flesh and white. I slap on the base colour, a base/middle flesh mix, middle flesh and finally middle flesh and white mix. So around four shades I guess.
One thing I do find helpful is to add a final highlight to the skin when I've finished the rest of the figure. There's usually one or two places where a well placed highlight makes all the differece. Good examples are ankles (on bare legs), wrists and one side of the forehead.