Surefire,
I used a basic tan flat latex house paint as an undercoat on flat areas as well as on rocks. The rocks were then given additional dilute washes of red oxide, raw umber, and raw sienna.
For ground cover, I used Woodland Scenics earth, yellow grass and a touch of burnt grass. I also used various colors of actual sandy soil, some gathered on a trip to canyon country in Utah.
I also found a great red sand in the pet store that is intended for a lizard terrarium. You can also get various red and tan sands at craft stores such as Michael's in the sand art section of the store.
Also, be sure to check out the neat assortment of desert cacti, etc. from Scenic Express (forum sponsor - click banner at top of page).
Good luck with your modeling. The great thing about modeling the southwest is that no matter how colorful you make the soil or how outrageous you make the rock formations, Mother Nature will always show you real life examples just as unusual.
Jim