I think the word Kryptonite should be capitalized, simply because it's a NAME. But I don't think that the "green" or "red" should be capitalized when they're used with the word Kryptonite, because they're just descriptive words about the Kryptonite. Therefore: green Kryptonite and red Kryptonite.
Of course, I have never really been consistent about this myself, so sometimes, I don't capitalize any of it at all, and sometimes I capitalize the green and red too.
And I don't know about anyone else, but I've seen "Oxygen" and "Helium" and other elements capitalized quite often.
"You take turns, advise and protect one another, even heal or be healed when the going gets too tough. I know! That's not a game--that's friendship!" ~Shelly Mezzanoble,
Confessions of a Part-Time Sorceress: A Girl's Guide to the Dungeons & Dragons Game Darcy\'s Place
