Red: Hellstone or brimstone suits it I think
Yellow: I think brittle gold doesn't really roll off the tongue, so maybe brittlestone or australuim, not quite sure
Brown: I reslly like the name Slipsand. Partly because the Nether was called the slip for about a day or two and partly because it slows you down and is like mud (which is slippery hence the "slip"?) lol Second idea is kind of a stretch but I like it.