I have one of their sulfur fields mats. I shot some basic colour (browns and orangey-reds, all super-diluted acrylics) onto it with the airbrush, then drybrushed up the details. Simple, but it can take a while, depending how perfectionist you want to be with the details.
Use cheap craft paints. Just make sure you don't cake the paint on; the mats take paint well, and it stays on well, so thin layers work best.
Quite a good job to do with kids, if you have them; it's fun and pretty hard for them to mess up.