I love the really stylized look of both of these guys. They look wicked; I think that the heavily restricted pallet really ties the figures together.
I would recommend buying a single pot of yellow (from GW or an art shop). Although you can't make every possible colour, one of each of red yellow and blue will allow you to mix many colours. You should be able to make your reds slightly darker (more browny) by adding a dash of yellow and blue instead of black. Similarly for the skin, a dash of yellow would make her skin look a bit healthier (browner).