pretty but unoriginal
It's a very standard concept and you haven't added anything new so your rating takes a big knock for that.
But you've made it very slick and nicely finished. I especially like the little movement details like the hop as the next ball jumps up into play.
Found a bug:
I fired a red ball off the wall and it bounced then landed on top of a green ball that was one step in from the edge. After I cleared that group the green ball was still there isolated and it didn't move down when the rest did. Two shots later the 'next ball' didn't hop up into the gun so I had nothing left to fire.
Overall very nice, have a think about ways to twist the game-play a bit though?