Interesting concept but difficult to control with a keyboard. I didn't know opposite colors could kill enemies until about halfway through. Also, if you hold ctrl while being hit, you can fly really far.
The Adventures of Box Boy
You are Box Boy. Some sort of evil has stripped all the colour out of the world and its up to you to try and restore even just a little.
You gotta change shapes to break the walls down and kill the monsters.
Controls: Arrows/WSAD to Move, Space to Jump, Control to Change shape, Space/Enter to select menu item.
Music by Jesús Lastra - http://opengameart.org/users/jalastram
Extract all the files from the ZIP folder and run Boxboy.exe