Here are a few minis I painted a little while ago for an Exodite force. The idea here was to make the wraithguard look worn and old.
To accomplish this look, I took a similar approach to the way that I paint fade leather. I made the bone color really varied, the deepest colors are greenish browns, with more yellowy brown and bone colors above that, and the highest highlights are in an off-blue color. The contrast between the different layers makes the bone look like it became worn differently in different spots.