Simple
Another perfect example of how simplicity is often the best route. As far as game design goes, this game is superb. It's rewarding to play the first time, as it is the hundredth. As far as improvements go, sound is definitely a must. You could also add some neat visual effects for when a square is correctly accounted for or not. Square squishes, and bounces off box if missed? Square explodes into a colourful particle flurry of awesomeness if correctly managed? Now if you decide to add neither of those improvements, please add just this one, make the Yellows brighter, and the greens darker. They're too hard to distinguish for my colour disoriented self. Also increase the lines on the box from 1 pixel to maybe 3 pixel. Overall this is a superb game. Nice thinking.