Bowser Rainbow Rumble is the third and final Bowser sub-level in Super Mario Star Road. This level appears to take place in either space or the sky. You can access it by having 80 or more Stars. The door is at the end of the Star Road.
There are 2 Stars in this level.
Star 1: Another Path
After jumping off the ramp to the moon-like platform, the path splits into two - one with a ramp leading to a platform, the other leading to the boss battle. The former will take you through a gauntlet of small platforms and Chuckyas to the Star.
Appears after Mario has collected 121 Stars! The Star Replica is on top of the pillar in the center of the tilted spiral pathway. You can reach the Star by long jumping to the top of the pillar from the very highest platform in the level located near the entrance to the Bowser fight.