To use Action Replay codes for Pokémon Colosseum on the GameCube, you would need to enter specific codes that modify the game's memory. For instance, to encounter Jirachi, you might use a code that alters the game's event flags. However, specific codes can vary, so it's best to search for a reliable code database or forum for the most accurate and up-to-date codes. Always remember to back up your save data before using Action Replay, as it can sometimes cause glitches.