Thoroughly clean and dry the wall you will be painting. Use primer, if necessary, following the instructions on the primer packaging. Allow primer to dry for the recommended period of time.
Pour the can of medium grey paint into a paint tray and dip your roller in the paint. Paint the entire wall using the medium grey. Allow the paint ample time to dry, according the the product instructions.
Draw castle stones on the painted wall using chalk. The stones should be about 10 inches by 14 inches and have irregular sides and rounded edges. Space the "stones" about a finger's width apart.
Dip a 1- to 2-inch brush into the light grey paint and paint the top of each stone in a back and forth, random manner. Work with three stones at a time.
Repeat Step 3 with the dark grey paint, painting the bottom of each stone, in the same sets of three you painted with the light grey paint.
While the paint is still wet, dab the light and dark grey painted areas with a wadded plastic bag to provide texture to each stone. The paints may mix a bit in the center of each stone, which is all right as it will give a nice effect when blended.
Dip another brush into the medium grey paint and touch up the areas between the stones which may have gotten the other paints on them. Allow the paint to dry.