Here's a quick tutorial on how I paint Grotesques and Wracks.  First start with a good basecoat.  I use black for my dark eldar.
Then I heavy drybrushed the whole thing this Brown color.  Brown is a good base for coppers and golds.

Next, drybrush the model with the copper.  This way it picks out all the little metal details.

I paint the flesh areas with the base Deneb Stone.  And wash with Ogryn flesh.

 Next, I started picking out the armor plates with the foundation, mechrite red.