If the screen appears only black, move the mouse around and hopefully you should find the scene.
Click on the game to start it, press 'esc' to pause and unlock the mouse.

The targets are worth:

Blue: +10 points
Red: -15 points
Yellow: +20 points, and a gun upgrade(faster firing rate or faster projectile)
Green: +100 points
Magenta: 50/50 chance of +/- 25 points