Super Mario Galaxy Tips

Prankster Comets
Fast Foe Comet = Orangish and will make the enemies move faster.
Cosmic Comet = Blue and will make you race a mirror of yourself.
Purple Comet = Purple and will make you collect all 100 coins to get a star.
Speedy Comet = Red and will limit the time you can stay in that galaxy
Daredevil Comet = Black and will give you a low life level in the galaxy.

With your knowledge on the comets know, you will be able to go into the galaxies with these comets in orbit.