Mario and the Rainbow Curse is the 2nd sequel to Super Mario Galaxy and 1st to Super Mario Galaxy 2.
After Bowser fell into a black hole, marking his death, Bowser Jr. decided to avenge his father by taking down the whole Mushroom Kingdom. Since his dad wasn't around, he didn't capture Princess Peach. He snooped around the castle looking for things that would be useful. He then found his magic paintbrush. Bowser Jr. called all the Koopalings to share the news that Bowser has died. He gave them bubble shields so they become immune to his attack. Bowser Jr. then took away all color (and life) from the Mushroom Kingdom, and entrusted the Koopalings with the colors. Morton Jr. was given Red, Larry was given Orange, Iggy was given yellow, Lemmy was given Green, Ludwig was given Blue, Wendy was given Purple, and Roy was given Gray. Soon after the attack, Rosalina came and used her Luma's power to make Mario, Luigi, Peach, and Toad normal again and sent them on their quest to save the world (again).
Worlds and Bosses
There is also a Rainbow Episode, where the Koopalings try to bring back Bowser, who is the secret final boss.
More to be added later.