When coloring your faux stones or walls you should work with a simple palette of colors chosen to match your scene. You will need to decide on the characteristics of rock found in the area (check actual photos of stone walls), or match the stone to the colors you choose for soil (worn stone is the basis for many soils). If you are working with existing miniature pieces (village buildings for example) you should tone your stone to match that of the village pieces. Department 56's Little Town of Bethlehem uses resin rocks which have squared, hewn edges in a basic color palate of gray green, raw umber, and light ochre.
Basic Color Palette for Stone
Your basic color palette should have three main colors, a dark, a mid tone, and a lighter tone. They can be the same color (dark grey, mid grey, light grey) or they can be a range of colors found in the stone in a particular area, dark grey or grey green for the shadows, rust brown or raw umber for the iron stained stones, and pale grey, cream or ochre for the lighter or worn areas. In addition to the three main colors you should also have a highlight color. This might be a raw umber to add iron staining, a very pale cream color to resemble sandstone or light quartz, or a green color to resemble lichens or moss growing on stone.
Mixing the Colors
For the simplest color schemes for stone, I usually use some flat black acrylic mixed into my gesso to make a dark grey for the crevices in dry stone walls. I don't mix the color in completely, as the variation of color mix in the paint gives some depth to the final effect. I will paint this coating into the wall cracks first, (see the samples for the styrofoam wall) then add white to some of this mix to make a mid grey which goes on the faces of the stones, and add white, or beige to the mid grey to make a highlight color which is dry brushed, or spattered on the stones to add some texture and break up the other colors. I may spatter or dry brush the dark and mid tone colors on top of the lighter color in some places to break up areas which are too flat. Using the dark grey base I mixed first as the main color, and adding additional colors to this base color to lighten and adjust it, keeps the colors in the same tonal range and is quite effective. When the main colors are dry, and the stone texture and color is where I want it, I will use a highlighting color to give some interest to some areas of the stones, but not all. I can use bright green or orange to resemble lichens or moss, or dry brush or spatter on a bit of light beige to give some stones more highlights.