Once your styrofoam is in place and you have shaped it, then paint the whole thing black.
I did most of it with a brush and it works fine. But I remembered that I had an air sprayer. Now, you can not use spray paint to paint bare foam. The propellant in the can will melt the styrofoam. But you can pour some regular paint into a paint gun and spray it on like that.
This sprayer was like $20 years ago at Harbor Freight hardware. It is designed to paint cars. It hooks up to my air compressor. An airbrush would have worked the same way.
It helped to spray paint into the cracks and crevices.
Once it is all black, I mixed up a bit of dark gray paint. I grabbed a stiff bristle brush and drybrushed some gray onto the rocks.
Brush lightly, hitting the highlights, leaving the dark shadow areas black. Slowly bring up the highlights.
It will suddenly look more like rock. But don’t worry, you won’t really see the magic until the very last minute. Through out the entire project all you will see is styrofoam and paint. Eventually enough layers will go on and your mind will stop seeing bits of the project and will finally see the landscape. Don’t give up yet.
Next I added some red paint into the gray mixture. Rocks are different colors, not just one shade of gray.
I added the red right into the paint bucket of gray paint. Drybrush it lightly on.
Leave the blackest shadows intact. Once the basic rock is painted, I added some green where moss/grass would grow. Imagine dropping grass seeds from the sky. Pick out flat horizontal spaces and dab some green.
Dab on some small spots of grass color. This is a little more precise than the other colors. Only add it where plants would grow.
We will add grass later, but the paint will help the illusion.
Go ahead, make a castle, have some fun.