Have you completed the quest for the Ruby and Sapphire items in your FireRed game? If you have, but you still can't trade to Colosseum, then that might be the problem. Have you beaten Colosseum? If not, then you won't be able to trade to any GBA games.
As long as you've done both of those things, there shouldn't be a problem. If you still can't trade even after completing both of those tasks, then there must be some deeper problem.