Excellent acomplishment
I enjoyed playing this game that you've created, it was very interesting to see how simple it was - yet made me continue to play for some time.
One thing of note, however. If you hit a red ball and lose, and there are presently yellow balls on the screen, they will remain on screen and even carry over to your next game if you hit the 'Retry' button. So essentially, instead of starting with the one green ball, you'd start with that one green ball and a couple yellow balls if you had them from your last game.