Before You Start Coloring and Finishing Your Faux Stone Walls
Make sure you have decided on an appropriate coating and three colors of "stone" plus a highlight color for your stone wall. (see step 4 and 5).
Fill the Crevices Between Faux Stones
Start with the darkest layer of your stone. This will set the shadows for the crevices between the stones and disguise the shape of the beadboard beads. Use a fairly heavy layer of foamcoat, gesso or grout, mixed to your darkest color, and apply with a stiff brush (or a palette knife for grout) to the entire wall, on all sides, including the bottom of the wall to help seal and protect it. This may take one or two coats (especially if you are using gesso). When you are finished the entire wall should be covered in your darkest color and no white areas should show through. Allow the coating to dry.
Disguise the Styrofoam Bead Shapes When the first coat is dry, apply a second layer to any areas where you can see strong outlines of individual beads, or seams in your foam. You won't be applying much more color between the stones, so the bead shapes need to be well disguised with your darkest color in these areas. Allow this second coat (if needed) to dry.
Add Mid Tones to Your Faux Stone Wall
Mix some white acrylic paint into your previous color or mix up your second, mid ground color, and dry brush or dab this color onto the top surfaces of the stones. Try to leave some of the darker color underneath showing. If you want stones that resemble granite, or quartz based stones, touch your loaded paintbrush to a piece of waste newspaper or paper towel to remove excess paint, then lightly touch the ends of the brush to the surface of your stones to leave small spotted or blotched paint sections. If you want striated stones that resemble shale or sandstone, use a stiff brush to spread your compound in one direction across the top surface of your stones, leaving strong lines of colored compound behind.
Don't worry if you get too much color in an area (see the section of wall on the right in the photo) You can always go back over the section with another coat of some darker paint to break up large areas where you applied to much paint.Set the wall aside to dry with its second coat of finish. The finish should be building up in layers on the front edges of the stones, adding some more texture and depth to the shapes you originally carved.