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)