Something I was quite surprised about but it worked for the skin was...
1. Undercoat with Chaos Black
2. Basecoat with Dark Flesh (thats the weird bit)
3. Use a darker green (think i used Catachen green) and relatively heavy drybrush for the raised bits, do not go into recesses (easy enough, you shouldn't have a problem).
4. A small drybrush of a lighter green (think I used camo green) over raised parts again, but this time only lightly, and only over the very peak of the raised bits.
5. Wash with Thraka green.
I found that gave me a VERY dark and grubby look, which is what I was going for. Takes no time at all as well.
The quickest method I have found so far is to get all your YELLOW paints and other paints which are some-what yellow (I think bubonic brown cut it, maybe :| ), then -
1. Undercoat black
2. Basecoat with one of the YELLOWS
3. Heavy wash of Thraka green.
I found the darker yellows worked the best.
Another method I liked was...
1. Undercoat Chaos Black
2. Basecoat Catachan green
3. light drybrush camo green over raised parts.
4. Wash with thraka green.
5. very careful highlights of camo green.
gives it a grubbier and dirty look, which I feel is more ork-like
Anyway I have more, but those are some ideas for you to work with mate.