Caps are hats that Mario, Luigi, and Wario can wear in the games. There a total of three hats in the original game - The Wing Cap, the Vanish Cap, and the Metal Cap. Yoshi doesn't have a hat, but he can transfrom into one of the three. Mario's hat is red, Luigi's hat is green, and Wario's hat is yellow.
"There are special Caps in the red, green and blue blocks.