Actually, Metallo has only ever had green Kryptonite, though there is a character named Reactron who had a similar design who had gold and remains unclixed. Last I read of either of them was War of the Supermen.

There was a story relatively recently (around 2008 or so) involving a Kryptonian child appearing from the Phantom Zone. He ended up being Zod and Ursa's kid, whom they used to see if they could actually break out. They do, and Superman ends up turning to Lex Luthor and his Revenge Squad (Metallo, Parasite, and Bizarro) for help. As had been placed in him during the annual occuring a few months before the story started, Metallo had a piece each of Blue, Gold, Red, and Green Kryptonite. There's a scene during the fight in Metropolis where he and Parasite have a good talk about his supreme Kryptonian-killing power sources, where he uses Gold Kryptonite to make a bunch of flying Phantom Zone escapees drop fifty stories or so to their deaths. It's actually a funny sequence, if you can find it somewhere.