Aquarium sand and grit, and leave it the natural colours. When you find a well stocked store, they will have sands and gravels in all kinds of colours and sizes. Also; they usually sell the stuff in handy bags of 5kg, so no need to buy 25kg bags at a hardware store...
I know it goes against the grain, as we're all used to paint and wash and then drybrush our sands and gravels, but trust me; the end result is so much more realistic when left in their natural colours.
Check Luke's APS Youtube channel for some results; he leaves his ground covers a natural colour most of the times and the results are phenomenal.