Start with our plaster-cast rocks in place, and the Sculptamold earth material worked around each casting to blend it in. Here, as in a lot of areas, there are many ways to do the rock coloring - here is my method. It's fast, easy, and for the first time, you really get a visual pay back for all your work up to this point. Trust me, I'm no artist, but it's really hard to get this looking bad.
First, stain the casting using Railroad Tie and Bridge Stain from MicroMark, full strength, with a cheap 1" brush. Work from the top down, letting it run to the bottom. Neatness doesn't count here; any areas around the rock that you get stain on will be covered by earth paint, and dirt. Your goal is to cover all of the white. The stain dries very quickly, and is absorbed into the plaster, so work around all your rocks in an area, then come back and do a second staining.
Next is dry brushing. Place a small pool of color on a scrap of cardboard, and dip just the tip of the 1" brush in the paint, then rub most of the paint off on the cardboard. It's best to start with too little paint than too much. I use cheap water based craft paints.
Start with white - this may seem odd after we just worked to hide all of the white plaster, but you want to just highlight the tops of the rock, where the sun would hit it. The next steps will tone it down.
Next, lightly dry brush on burnt umber over the entire rock, toning down the white highlights, and adding a reddish color to the rock. You can see how the underlying stain color still shows through; we're adding layers of detail.
Last is raw umber, the darkest color. Dry brush on the same way, but mostly at the bottom, and on the underside of overhangs and ridges - "where the sun don't shine". I then paint on a bit of full strength raw umber in the deepest cracks and crevices. Be careful with this, still not too much paint!
That's it! Remember, after the basic dirt ground covering, lots of additional layes of color and texture will be applied around the rock - and maybe even some moss and small drips of water on the rocks themselves. I've seen other colors used, for rockwork, too - blue-greys, other reds, even yellows - whatever gives you a pleasing look to the rock.