Super Mario Galaxy/Prankster Comet

Prankster Comets are unlocked after you have 12 stars. They make galaxies crazy in different ways depending on the color of the comet. There are 5 different types.

Speedy Comet
These red comets make you complete a level in a certain time. This time depends on the level e.g. Dino Piranha from ../Good Egg Galaxy/. If you are struggling, try using lots of long jumps.

Daredevil Comet
These silver comets give you one life for a level. The challenge is usually to beat the boss, with the exception of Lava Spire Daredevil Run from ../Melty Molten Galaxy/. This can be extremely challenging (take Bouldergeist from ../Ghostly Galaxy/).

Cosmic Comet
These blue comets make you race another Mario (or Luigi). Cosmic Mario is easy enough (take ../Freezeflame Galaxy/. Here Cosmic Mario follows a preset path, where you can take shortcuts). No, Cosmic Luigi shows us what hard is.

Fast Foe Comet
There are only two galaxies which have this comet (../Beach Bowl Galaxy/ and ../Toy Time Galaxy/) which is just as well, because these yellow comets, as in the name, make foes move twice as fast. It's good if you can long-jump.

Purple Comet
You do this once in the ../Gateway Galaxy/ before having to beat the storyline to unlock this comet. The color need not be mentioned (it's obvious) and you have to collect those little coins. Sometimes you have a time limit.