In Null Chroma, you use color as a weapon.

When an enemy is hit with a projectile, the color of that projectile is subtractively mixed with its color. In order to kill an enemy, null its color to black.

The goal is to null any enemies to prevent them from reaching your orb (the flashy white thing) until time runs out. If more than 10 enemies reach your orb, you restart the level. The orb's health is indicated by its color, white is good, black is almost gone.

There's a friendly little ring around both your orb and the enemy spawns indicating how much time is left in a level. Keep in mind that the timer counts down to when enemies stop spawning, they still all have to be nulled for the level to be won.

Controls are explained in game, the only thing you need to know is that the original requires a middle mouse click to fire green, whereas the version marked "I have a cruddy mouse" will fire green if left and right click are both held down. I think this edit falls under the porting exception, but I don't want to push that.

Lastly, those three circles at the bottom of the screen show the RGB value of whatever your cursor is pointed at.

Have fun! And if you enjoy this game check out my gamedev blog: https://www.tumblr.com/blog/deltadaedalus , I make lots like it!