Rainbow Note Block

Rainbow Note Blocks are rare blocks which lead to secret areas, before returning the player to where they left.

Super Mario Bros. 3
The Rainbow Note Blocks do not actually display rainbow colors in the game. They look like a light red Note Block. When jumped on, the player will be propelled to a secret area, a Coin Heaven. This area is filled with coins, and occasionally hosts a 1-Up Mushroom.

Super Mario 3D Land
In certain area, Mario or Luigi must locate one of the Rainbow Note Blocks in order to gain one of the three Star Medals. When Mario or Luigi arrives in the secret area, they will get a Rainbow Star and have to race to the other side through many enemies. If they do it right, they will gain a 1-Up Mushroom in addition to the Star Medal.

Trivia

 * In Super Mario 3D World, the Cloud Cannon replaces the Rainbow Note Block. It leads to a secret area which is the same format to those in Super Mario 3D Land.