The Sphere of Light, according to the mysterious voice that guided Wisp, the only way to restore color and happiness to all of Rainbow Land.
When Wisp was brought to Rainbow Land, the mysterious voice guiding her told her to seek out the Color Belt and the Sphere of Light. So Wisp set off, in search of the belt and the Sphere of Light. As it turned out, the sphere had taken the form of Baby Bright, whom Wisp found shortly after meeting Twink.
During Wisp's confrontation with the King of Shadows, Baby Brite took the form of the Sphere of Light appeared and aided Wisp in the battle against the King, taking the form of a ball of light as a means to defeat him. After Wisp triumphed through the power of the Color Belt, Baby Brite vanished, presumably restoring color to all the land.