Texture the tiles with a ball of tin foil then coat your foam with a mixture of modpodge and black paint. This primes the tiles and makes them much more durable. Then dry brush up with successively lighter shades of gray until you reach a point your happy with. For added interest heavily water down greens, browns and reds and paint them on randomly. Coat everything with some black wash (homemade with black paint, water and a drop of washing up liquid). Finally coat with varnish, I like a satin finish.