Planet Rainbowl is the first planet on the road from the Star World to Zythurvia, and it had a Crystal Comet Medal and a Zyther Crystal. It's textured in the 7 rainbow colors, and has appeared in MarioMario54321's Super Mario 64 Bloopers movie The Cosmic Crystals IV: Zythurvia.
It's mainly a recolored/reversed version of Bob-omb Battlefield and the Vanish Cap Course. The boss is the Rainbow King, who would only give up the Green Zyther Crystal under "tick time", which is under the time limit of Rainbowl's Crystal Comet mission.
The music was the Unused Planet soundtrack and the Flipsville Galaxy themes, both of which are from Super Mario Galaxy 2.