Freddo: Welcome to the-gauge!
Shamus is a very sharing person and a lot of his techniques are described in the academy. I don't recall anything on the rock coloring right off, but there sure is some good reading there! You get there by clicking the "the gauge" icon at the top, then scroll down to the bottom of hte front page, you'll find the academy under archives. He also has a lot of info on his website, which you can get to by clicking the image at the bottom of one of his posts.
Looking forward to seeing some pictures of your work soon!