I love those bases
The thing you need to do when working on bases is to pick colors that provides some contrast with the model, but not colors that are too distinct that draw attention away from the model.
The natural colors of the Runic Mountain bases are browns and greys (because we're talking about rock). I would avoid the greys, because they'll just blend in with your models. Therefore, I would pick colors similar to what
GW uses for their generic bases -- the light tan.
The formula for the generic
GW "brown sand" bases, I believe is -- basecoat Zandri Dust, wash Agrax, drybrush Tyrant Skull, drybrush Pallid Wych; base ring of Steel Legion Drab.
Allso a tuft or two would really spruce them up. I might try dark green (swampland) or white tufts from army painter. If they aren't by a rock or some other large object, I'd trim them down a bit.