Gretchin or Grotz (take your pick)
A newer and more complete guide to painting Grots.
I do like the underdog and this page shows how I painted my Grots for Warhammer 40K.
Note that this is a topsy turvey guide in that some of the new pictures are in fact of new unpainted grots.
For Orks I use black, for Gretchin and Grots I use a white undercoat as it usually results in a lighter look for the Grots and a darker look for the Orks.
Once the white undercoat is done, the skin tones are painted in Goblin Green.
The next stage is to highlight with Bilious Green.