The Rainbowtastic Temple is the first area in the Moon World. It is where the 1st Lanu Orb, the Rainbowtastic Orb, was found. It is also where the Moon Tablet was found. LuigiLuigi09876 got corrupted by the orb in this area, and MarioMario54321 had to fight him out to get him back to normal.
The area is similar to the Lunar Eclipse videos from MarioMario54321, possibly because the orb within holds the spirit that caused the flashes during the Lunar Eclipses.