When you turn on the game and the words "Game Boy" show up on screen and you hear the chime, press buttons on the D-pad to change the color of the game.

When you collect all 5 coins and are ready to enter Wario's castle, try to get the bunny ears before you enter. It will make the stage easier. You can use them to fly over mace balls and fireballs, and can land on moving platforms easier. Also, you will have the advantage over Wario because you can fly above him and land on him to do damage to him.