Good game! I like the simple mechanics and minimalist design.
There's just a few issues I would like to point it out:
- Although green and orange colors are easy to identify, their counterparts, lime and yellow, can be hard to see and differentiate between then. You could try making them brighter or using a different color.
- Early game is kinda slow, and difficulty doesn't seem to increase. It would be nice if objects became faster after each "level" (when the circle change colors) and had different movement patterns. That would definitely help making the game more challenging, and fun to play for longer periods of time.
- Not really a big issue, but... in addition to the "Play" button, having a key (like Enter) to press would be nice to have, too.
Other than that, the game itself is fine. Given the resolution, it's got a fair amount of visual effects, which helps to give that great "pixel art" feel!