Definitely a cool concept, but I do have a few criticisms.
First of all, horizontal movement is a little slow to kick in thus feeling a little irresponsive, which worsens the fact that it's hard to tell which cube is the player. Also, due to how the enemies were oriented, it took a very long time to figure out that you're supposed to kill the boss from above where the health bars are. Second killing the boss is very difficult, and if there's a certain method you're supposed to use to kill it, I couldn't figure it out and didn't end up beating it. Third, I don't know how to replicate this, but I encountered a glitch where I was immobilized on a spike, not taking any more damage forcing me to reload, but I have no idea how common that bug is.
Besides that though, the shooting felt pretty good, might benefit from having incentives for not spam clicking on the enemies since it destroys bullets (good job on that though,) Once I found out about it, the gravity flipping feels really good, and it was a really fun mechanic to mess with. Also the jumping and double jumping felt really snappy and responsive (as are the other feature I mentioned in this paragraph,) and felt like it was a really reliable movement, just with it used spacebar.
Overall, for a game made in a day, and only using two colors, this thing turned out great.