I get asked this alot!
Believe it or not it started life as a old white sheet that was going to be chucked out. I sprayed it with various coloured car primers (Black, grey, yellow and red oxide). When it was wet I threw dirt on it to add some texture. I went over it with the colours again but at a distance to get a overspray effect.
As time has worn on I add some more colour when cleaning brushes of browns and greys. These is some cheap tester pots from the DIY store in there too - that is where a lot of the browns have come in.
The reverse is simply grey for urban games. It has a road network painted on it in cheap black poster paint.