We typically suggest Yellow Ocher, Burnt Umber and Black for generic rock coloring, however different geographic locations have rocks of different color so choose the pigments that best represent the area you are modeling.
The Leopard Spot technique is our favorite method for coloring rocks. It is a simple technique that yields realistic results. Start with the suggested dilution ratios. If darker colors are desired, add additional pigment to washes.
-Dilute 2 different secondary colors, 1-part pigment to 16-parts water. Color suggestions: Yellow Ocher and Burnt Umber.
-Dilute a third pigment for a dominate color, 1-part pigment to 32-parts water. Color suggestion: Black.
Start with the lightest secondary rock color. Using a foam brush, dab first color randomly over 1/3 of rock. Rinse brush. Dab the next secondary color randomly over a different 1/3 of rock. Rinse brush. Leave approximately 1/3 of the rock white.
Brush and dab the dominate Black Wash over the entire rock face. The Black Wash pulls the colors together. When dry, seal rock colors with Scenic Cement (S191) and let dry. Then, brush on a less diluted Black Wash (1:16) to accentuate rock detail. Spray again with Scenic Cement to seal.