I use Vallejo Pumice which comes in different colours > white, grey, earth brown, black, desert sand etc.
I started using grey but then you have to paint it black and colour it. Now I use black and go brown over it.
You can sprinkle sand or little slate chippings on top of it as it's wet so give it texture for when it dries.
I bought from the art shop some cheap synthetic paint brushes and a big tube of black paint and one of brown, both acrylic. Those are your base colours and I sloppily drybrush it grey in patches then paler brown over the rest, sometimes after a wash of Army Painty tone ink.
I have some desert WIP ones but they haven't turned out brilliantly so right now they are buried in a big box until I can work up the energy to redo a desert board.
