Hi there. I have a method to paint skin that is pretty good, below is a pic of a wych elf I have done for my venom crew, that uses this method:)
I start with a base of dheheb stone (old foundation paint) followed by a watered down wash of purple wash (you dont want the recesses to be too dark, so about a 50/50 mix will do). Then re highlight with the dheheb stone (everywhere except the recesses). Mix 50/50 dheheb stone with skull white and do a lighter highlight on the more raised areas. Finally a pure skull white highlight on the very raised parts. Job done
hope that helps. Also if you want to convert the paints I mention into new gw paints see this chart http://www.dakkadakka.com/wiki/en/Paint_Range_Compatibility_Chart