I thought this was really clever. My main complaint is the use of color. In the 2nd and final levels, the enemy releases small red squares as an attack. At the same time, your ship and bullet releases similarly-sized "dark" yellow squares (dark due to the transparency on black). I am colorblind, and I struggled to be able to quickly identify if a particle was from me or the enemy. I could figure it out by looking at the particle's behavior, but that takes more time, forcing me to be more cautious than necessary. It would be nice if the particles were more easily distinguished by more than just the color (eg. changing brightness, shape, or texture).