How do you use the rainbow pass in Pokemon FireRed?

You can't use the "Rainbow Pass" in Pokemon FireRed. There is no item or feature with that name in the game.

It's possible you're thinking of something else. Perhaps you're thinking of the "Rainbow Badge" in the game, which is obtained by defeating the Gym Leader in Celadon City, Sabrina.

Could you clarify what you're trying to do with the "Rainbow Pass"? I might be able to help if you tell me more about what you're looking for.