You need an answer You can't catch Celebi in Pokemon Colosseum! you need to go to a Nintendo event with Pokemon emerald/ruby/sapphire version(s). alternativly, you could use a cheat system. from 07meowth.
You can only get Celebi in Pokemon Pearl if you migrate it from Pokemon Ruby, Sapphire, or Emerald, or if you use an Action Replay code. If a friend or someone on Wi-Fi has it in the Gobal Trade Station, you can get it from there. You cannot catch Celebi in Pokemon Pearl without attending an event.