Pokemon that are catchable in Colosseum do have business being in a Master Ball, for the record. There was an easily exploitable glitch in that game that allowed one to use Pokeballs without losing it. I've taken advantage of this glitch myself numerous times. It's a lot of fun when you get the Master Ball. But since you get the MB late in the game, there are only a few Pokemon that can be caught with it.
Actually, come to think of it, since Colosseum gave second chances to catch each of the Pokemon, you could theoretically catch every Pokemon in the game with a Master Ball.
But yeah, you can't get Swampert in Colosseum or XD.