Painted Boltgun Metal in a heavy drybrush over the metal parts. I intentionally let some of the Tin Bitz show through so as to represent rust, etc.
Then I started finishing up one of the boyz, and when I went to do his eyes, I accidentally got some Blood Red on his left hand. Rather than touch up and fix the mistake, I decided to go with it... This particular ork just blew somebody away at close range, and got a face full of it.
Normally I don't go over the top with blood and gore on my models, but rarely, the model seems to ask for it.