Rainbow Mario is one of Mario's power-ups. It appears in Super Mario Galaxy and its sequel, Super Mario Galaxy 2. It can be obtained by grabbing a Rainbow Star. It is the equivalent form of Invincible Mario in many games where the Super Star could be confused with a Power Star.
It makes Mario invincible. He can dash into enemies without hurting himself but destroys them and can pass through any other obstacle. Also, if Rainbow Mario keeps running withouut stopping, he will start to run faster than usual.