Mate, those are absolutely amazing. You have to tell me what kind of colours / technique you used for the sand - it looks convincingly like a desert instead of a beach. =D
Actually that is pretty easy to do and very quick:
1. Basing material is Vallejo White Pumice (fine sand + acrylic medium; you could have the same result with sand + PA but you would have to do 2-3 layers to have an irregular surface while with White Pumice 1 layer is enough; and it dries white so you don't have to paint white
).
2. Basecoat with a SINGLE layer of well-thinned Vallejo Ochre-Brown (the sand grains should still be almost white when dried); the paint shall run but still be pigment-rich (like milk), do not hesitate to put too much paint so that it run out of the base.
3. Second SINGLE layer of thinned Vallejo Ochre-Brown (milk-like again); while it is still wet add some spots of Vallejo Parasite Brown (or GW Vermin Brown; or any red-orange brown you like) and help both colors to merge to have a smooth transition (if your Ochre-Brown is thinned enough that would be a matter of seconds since it is not dry).
You are done
The critical part here is to thin the paint properly so that it will gives a nice color and still keep the white of the basing material on each sand grain (thus no dry-brushing is requirement, and the result is smoother).
