I don't know of any tutorials, but typically, you don't shade black, black is as dark as it gets. However, to break up the monotony, grab yourself some Adeptus Battlegrey and do some light single-line highlights along the sharp edges of the armor. That'll really make it pop.
All of the green parts can be washed with either Devlan Mud wash or Thraka Green wash (both Citadel paints from
GW). Use the Devlan Mud if you want a really grungy look, use Thraka Green if you want the green parts to look... Well, green.
To wash the fleshy parts, pick up some Ogryn Flesh wash, apply and let dry. After it's dried, you can re-apply it in the areas that you want to darken up, such as the eyes, under the chin, etc. If it looks too dark, take the Tallarn Flesh you've got and do some single-line highlighting along the tip of the nose, chin and over the eyebrows, just for a bit of contrast.
You can see the techniques I mentioned in the photo of my Termy Librarian below, even if I used different colors for this particular guy: