I have had jet black tank gravel, but unlike
@Magnum Man I have always set up so I can get my long ape arms into every corner of the tank. With local white sand, I get cyano, I suspect because of silica. My pool place has brownish filter sand, and my old pool place had dark grey filter sand I really liked the look of. I tried colour alternation, but even before water changes, the fish and filters caused enough movement I got salt and pepper gravel from it, which turned out nice.
I wouldn't do the star trek design as I like red fish, and we know what happened to ensigns with red shirts.
I use 'sand' and 'gravel' for the same thing, really. I can mix finer playground sand with slightly coarser pool filter sand sometimes. It can give a non uniform colour I like. I don't use it with my homemade undergravels, which get pool gravel mixed with a slightly larger chemically inert river gravel.
There are enormous amounts of sand around here, just begging to be rinsed and used. I'd need an off road vehicle to carry it though, and would rinse away half of it. I live in what was the mouth of a wide prehistoric river, and in many places there's a thin layer of topsoil over obvious river bottom, even after all this time. If for some reason you dig 2 meters down, it's sand all the way down to the bedrock. But I've never wanted to own my own aquarium substrate mine, and I can't train the groundhogs.
