In case you were wondering, here's how to earn the cheat codes.

Fruity Colors Mode:Complete level 1
Pen and Ink Mode:Complete level 2
Gouraud Mode:Complete level 3
Big Hands and Feet:Complete level 4
All Map:Complete level 5
All Guns:Complete level 6
Big Head Mode:Defeat level 4 boss
Tiny Mode:Defeat level 5 boss
Infinite Ammo:Defeat level 6 boss
All Special Items:Defeat Primagen
Invincibility:Defeat Primagen
Infinite Lives:Defeat Primagen (on hard)