I REALLY like this game. It's a tough, impulsive puzzle/reflex game that somehow manages to fuse the gameplay elements of a rhythm game with the speed and thrill of "impossible" games like Super Hexagon. The visuals are gorgeous, and the interchangeable themes keeps a good variety going when I want it. However, there are two things that I feel the developers at Rlyeh should do, and I feel if they do this, then their game could be could be considered perfect.
1: there should be an option to select a certain color scheme. I know as the game progresses, the colors increase over time, but I did find myself having a lot more difficulty recognizing certain colors when they were close to other ones (for instance, I had the worst time differentiating purple and magenta from each other when they appeared at the same time, same with yellow and cyan when they were on a two-color ball). I had the best time with the RBY mix usually, and sometimes the MBC mix. I shouldn't have to let my ability to recognize the balls be left up to chance. And on that note:
2: include a colorblind mode. The main colors that are considered colorblind safe are White, Black, Brown, Cobalt Blue, and Yellow. I know the background for the game can alternate between black and white, so allow White to be Black on the "day" theme and vice versa on the "night" theme. I'm not terribly far into the game right now so I can't say how many colors end up being used per stage later on, but a colorblind mode would be absolutely vital to getting the most people into your game, as well as gathering respect from people who want to play your game but currently cannot.
I hope you take my advice into account!