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.