Let the music play!
The IVs and nature are certainly changeable in this game. I've taken the time to soft-reset over many months at one stage, and managed to acquire a good Hidden Power Electric Suicune (27/x/16/31/31/0 Relaxed, Hidden Power Electric 61). Why I'm having such a hard time getting a crit out of Umbreon, I cannot say just yet, but I'm reminded of the skewed RNG of the Battle Tower and Battle Frontier, which favours the AI with more crits, so I am wondering whether the odds of anything in Colosseum is not exactly the same as in the handheld games. This is another possibility I am working under.