I really liked the idea of each dimension affecting only itself; once I understood that I can dodge bullets by going to the other dimension, it clicked for me. Before that, it just seemed impossible to dodge everything. But really, it's about managing the amount of any one colour on the screen at a time. Clever!
The controls were a bit hard to get used to (also, I couldn't use dash; I used it once, and then I could never use it again for some reason, even after I reloaded the page). Also, in a bullet hell, standing still to shoot seems problematic, so maybe not having the aim lock you in place might be good?
Not sure about the melee attack; it seems nice to have something you can use if you're out of bullets, but I ate several bullets before getting near any enemies, so maybe it would be better to give the player some ranged options instead, or focus on making the gaps between reloads as seamless as possible (which you already do pretty well with being able to reload while moving and the dash reload on a kill).
Overall, a pretty nice game; well done!