Thats a really good looking nob you got there, well done!
I've painted quite a few orks since I started wargaming, so I guess I can give you some advice....
When painting skin, I usually try to give it a very muscular look. This can be easily archieved by painting many thin parallel stripes on the arms, ike this:
Also, almost every color I slap on an ork has at least a little brown mixed into it (if it isn't brown to begin with) or gets a brown wash. That give him a dirty overall appearance, which fits orks very well.
Edit:
oh and lets toss in this tut I made, about painting ork metal...
http://www.dakkadakka.com/dakkaforum/posts/list/439711.page
and
I wouldn't use devlan mud on the skin. Just mix your green with some browns if it is too in-your-face-green, or just use one that already looks dirty. I always wash with thrakka green though, it helps blending the colors...