I'm pretty sure that traditional D&D canon has adamantine being black, but I'd guess that any dull metal color would work.
If you're doing it fast and easy, I'd paint it black, heavily drybrush any surface not in complete shadow with a dark grey, then drybrush again with a metallic steel color on the parts that are obviously getting hit by direct light. Finish with a black wash and some random splotches of different-colored washes in random places for moss, rust, etc.
If you're really going crazy on it, I'd do the whole thing in non-metallic metal technique.