Grover asking Brian if Brian is "on the bus" is a reference that goes back to Ken Kesey (One Flew Over the Cuckoo's Nest author) and the Merry Pranksters as documented by Thomas Wolfe in The Electric Kool-Aid Acid Test. During acid trips you were either "on the bus" or "off the bus". The bus is not a direct reference to the psychedelic colored bus Further.