This is a generally a very polished game with a lot of artwork to support it! Here are a few things I like:
1) The simplicity/approachability of this game - it makes it easy for anyone to jump into and get a feel for the game. This is a unique rendition of those one-button approach game and I quite enjoy it playing it.
2) Transition between rewind to normal time. I love the fact that you made the balls move slower in rewind time - I believe that is a good small addition that makes rewinding so much more smoother, feeling more consistent and gives more control to the player in this game!
3) The small camera zoom-in on where you ball touched a triangle - for a game about juggling multiple balls (not literally), directly giving focusing the player onto where they messed up is good essential polish for a game like this - and I like it!
The game is actually quite well made/executed with its simple idea - I absolutely love it! If anything, I would like to suggest a few polish - they may or may not be integral to your design philosophy:
1) More sound effects maybe just to round up the game - such as in the main menu UI etc - on death, when a green ball spawns. I wouldn't be sure if it would be good but you should try toying around with it in your free time. What I mean by it might not be good is that it may add too much audio clutter - we'd want to avoid that.
2) Some more content. This is a very bare-bones answer but I think, if you plan on continuing this game, you shouldn't add more mechanics. Instead, you should derive from a content perspective and make more levels instead. Your simplicity in this game is a good strength and you could exemplify that. May I suggest: you could try considering mapping tools to make custom tracks. Who knows - just a fun thought.
Absolutely loved your game and dare I say, you have earned my admiration. I hope to someday make a game with your level of detail, of execution and of fun!