I still think the Colt's list of five are just the four archangels and something else. God and the Darkness are certainly not part of creation, and Death arguably isn't, either. We should note that the only other thing that got shot by it and didn't die was John Winchester, though.
But, really, by this point, the Colt has lost some of its mystique because we've already seen so many "kills anything" weapons:
The Colt
Sword of Archangel
First Blade
Hand(s) of God
And now the Lance of Michael, too. Many of them have already failed to kill something, and most of them aren't going to be tested against the stuff we want to see if they'd work against, like Leviathans (if they even still exist).
EDIT: Oh yeah. I just remembered that Alastair shrugged off being stabbed by Ruby's demon knife, too. He's definitely up there in power.