Enter the following at the title screen.

Effect	Code
Black and White Color Scheme	Die while holding L + B to start playing the game in a black and white color scheme.
Introductory Level Skip	Press Start + Right + Y.
Invincibility and High Jump	Hold Right, Y, L and A, then press start. During the game, press L to become invincible and R to jump higher.
Play as Miniature Ardy	Place the cursor on Game Start in the Presentation Screen, then press Up + X + Start (in that order).
Sepia Color Scheme	Die while holding L + A to start playing the game in a sepia color scheme.