I like this, although I think for the competition rules, it does require fast presses and reactions at times. A few possible improvements.
1. To restart the game, don't make the player have to click the centre box. Perhaps pause for 2 seconds, then allow the user to click anywhere to restart. Also, if you have SPACE BAR as your control, you should be able to restart the game using SPACE BAR too.
2. Sound effects can be nice to boost comprehension. That said, I do personally like the quietness of the game.
3. Perhaps a speed control, to make much easier for players with slower reaction speeds.
Quite an addictive little game.