Thanks for all the encouraging comments.
Jim ~ There is so much literature out there you should not have any trouble locating a good source for methods and techniques. As far as rock color, with all the granite here, I think light grey would be a good northeast rock formation base color. With ideas from different sources and with practice I start by making a very watery wash of my base color in any latex or acrylic paint. Liberaly brush the mixture onto your rock castings. The watery mixture ensures that the paint will soak into the plaster and flow into all the crevices. This also ensures that you will not wind up with a patch of stark white plaster. Next using a more watered down mixture of black paint I generously apply this solution and before it dries I wipe off the excess thus exposing and highlighting the formation. I've also used an india ink solution as some do. I get the same results with the latex or acrylic paint and it washes off your hands a lot easier.
Now the hard part, accenting the formation. Hard in that you cannot envision the end results and think you are ruining all the preceeding work with each color you add. I use the cheap acrylic paints that come in the little 2 oz. bottles available in most craft stores. Using a 2" junk brush I dry brush different colors onto the formation. The key is using a very dry brush, swiping the brush and even jambing or jabbing the bristles into the formation. It was my wife, my strongest critic, that pointed out the use of purples, whites and silvers in coloring the rocks. I was sticking with the browns, and tan colors and when she dry brushed on the purple I thought everything was ruined. Wow everything started popping as she added more and more colors. Should point out that good lighting is also a very important ingrediant when coloring. Hope I was able to help somewhat.