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 PathEdit
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.
Star Replica: Star Replica On top of the pillarEdit
After getting the first star of the Hidden Palace Finale, Star Replicas appear in every secret level in the game. One of them is here. It is on top of the pillar, which is revolved around by the tilted pathway.