One easy way is to put down a medium tan layer once you've primed the case. From there, apply light patchy areas in darker and lighter colors (medium brown, ivory, etc), very lightly, from a distance so that the paint atomizes and almost dries before it hits. Get about four or five colors and swap back and forth. Dust them from a distance in very light coats - about two to three feet away should be sufficient. This will give you a colored, speckled look that will seem uniform from a distance but will show the sandy grain up closer. It also gives a very rough paint texture that should help create a good matte finish. Go over this with your clear coats once you've finished it off, but this is one paint job you won't want to sand much, as if it is too glossy it will ruin the effect.
I did this on an entire sheet of plywood once that was used as a desert terrain gaming board for Warhammer and the like. It provides a very good effect. I'd recommend practicing a bit on a piece of scrap to get the hang of it, but once you get started, you'll see how easy it is. Don't forget to add a touch of black in, dusted lightly, to add a good range of color, but you probably won't want to go full white - a bleached bone ivory color should be about the lightest you'll want.