From the Super Mario Wiki
World Flower-Rainbow is the rainbow level of World Flower in New Super Mario Bros. 2. To access this level, the timer must end with the two digits "88" (that is, "x88", where "x" can be any digit) when the player reaches the flagpole of any World Flower level.
The level begins with Mario beside the Cannon Pipe in the generic world. Upon entering the pipe, two, columns of coins will appear and Mario will be shot to the sky, collecting the coins. Mario will land on a rainbow-colored platform. First, sets spinning coins arranged in the shape of a fan can first be found. Some coins will then jump up from the abyss below. A Red Ring can then be found, where eight parachuting Red Coins will then appear. During this time, a Giant Coin will fall. Finally, more spinning coins can be found before Mario jumps down and the level ends.