This ? Panel can be reached from the Toad's Factory start line. Fast travel there, then turn left past the start line to find a Yoshi's Diner selling Yoshi Snacks. Behind it will be a sloped blue wall ...

All Characters that require playing Toad Rally, require a certain amount of different color Toads, then you build their house and then you may play as them. Simply beat every level of the "World Tour" ...