This method uses the following colors:
Black Grey from Vallejo Model Color
Codex Grey from GW
Badab Black from GW
Whichever black primer you use. I use Board To Pieces black spray primer.
Over a black-primed model, I painted the armor Black Grey, being careful to leave some of the black paint showing in the recesses. Codex Grey was used to apply an edge highlight. Badab Black was washed over this to tie the colors together and bring out the details.
Here's the model when finished, he's from the Angels of Vengeance chapter.
This method is very easy, and I like the depth and shading that it adds to what is usually a very stark and flat armor color. You can check out the full tutorial at my blog: indy40k.blogspot.com