Rainbow Mario

From the Super Mario Wiki

Rainbow Mario is a special form of Mario that appears in Super Mario Galaxy. Mario can reach this form by getting a Rainbow Star. As Rainbow Mario he is invincible, much like when he gets a Starman, but he is also faster and leaves a light trail behind. If Mario doesn't stop running while in this form, he'll get star bits, but if he doesn't he won't get any star bits.

Rainbow Mario's theme is the same theme of Wing Mario from Super Mario 64. The music as well as Mario himself will increase in speed while in use, making Mario get harder and harder to control until it runs out.

Super Mario Galaxy - Mario Forms
Boo Mario | Bee Mario | Fire Mario | Ice Mario | Spring Mario | Flying Mario | Rainbow Mario

Mario's Forms
Bee Mario | Boo Mario | Cape Mario | Dr. Mario | Fire Mario | Flying Mario | Frog Mario | Hammer Mario | Ice Mario | Invincible Mario | Mega Mario | Metal Mario | Mini Mario | Plane Mario | Rabbit Mario | Raccoon Mario | Rainbow Mario | Shell Mario | Small Mario | Spring Mario | Super Mario | Superball Mario | Superstar Mario | Tanooki Mario | Vanish Mario | Wing Mario