It's pretty crude, but when you've got several big units to get through I find that's the only way.
All the paints are GW and are all now replaced with newer versions, but you should be able to find similar colours out there, or use the GW Paint Conversion Chart
The model is a slightly converted plastic Arrer boy from the venerable Warhammer 6th edition starter set.
Starting from a black undercoat (I used Boyes car primer)
First paint all the skin areas with Orkhide shade
Then overbrush the skin with Knarlock green, leaving the Orkhide showing in the recessed areas
Next, wash with Thrakka green