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The game is good. The gameplay is also good, but as for the graphics, I would recommend using color palettes, both ready-made or created by you.

Palettes? What do you mean? The bullets are using a "colorize" function rather than a palette if that's what you meant.

They mean sticking to a set of colors that are known to mesh well together and create a cohesive aesthetic. 
https://lospec.com/palette-list
These are generally for pixel art, but you get the idea.