All options
While the game is starting, press A,B,A,L,SHIFT A,Z.

More color
At the players select screen, press Z,B,A,Z,A,L.

Play like Gargos
At the players select screen, press Z,A,R,Z,A,B.

Sky stage
To have the special sky stage as a vs. level when you pick your characters, both players hold down then press down C to choose your character.