There was a 1e-era Dragon article that described mithral as silvery. Adamantine was described as a metal with hints of color in it, possibly blue, green or black or something like that- I don't recall the exact colors suggested. But adamantine, according to said article, comes in various colors.
Then there's orichalcum, which I always see as a sort of redder copper color, Baatorian steel (greenish), obdurium (I don't think it has ever been described- it's in the 3e Stronghold Builder's Guidebook) and probably some other exotic metals out there somewhere.