The color of rock varies enormously from place to place in the US. Limestone is light gray, coal is almost black, the sandstones of the southwest range from cream color to reddish brown. I suggest you look around for photos on the net. Pick a location whose rocks look interesting to you. Then match the shape, fracture patterns and color.

Some of the rocks in your photos are dark gray, which is OK for some areas. Rocks I've seen with that color include slate, schist and granite. Your other rocks are yellowish tan, a color usually associated with sandstone. Some of your rocks have horizontal cracks which look like sedimentary rocks, others look to me like eroded volcanic basalt.

Slate, limestone and sandstone are sedimentary rocks, so they are found in layers. Sometimes the layers are tilted & folded. This is very visible in the Front Range of the Rocky Mountains near Denver.

Igneous rocks come from volcanoes - they usually look like they were melted and poured into shape, then eroded. Metamorphic rocks start as sedimentary rocks, then get transformed by heat from volcanoes. They sometimes are layered like the original sedimentary rocks.

The best model rocks are shaped and colored to follow a particular kind of rock, or related rocks found in a single area.

Frank, try and lighten up the dark areas with a lite whiteish greyish wash. Very lite coat of wash. of to much. Try one area, see how it looks after drying, then go from that point forward...

Thank you for the help

Frank