What is the best colour to use to paint the walls of a small enclosed backyard?

A pale stoney color (gray or brown) would probably be best. Better yet is planting some climbers on those walls so that the boundary become less defined. Found a site (below) that has some nice ideas for the smaller garden. With such a private and enclosed space, I'd try my best to think of it as a room and approach its layout and decoration in much the same way as one would an interior room.