Damn this game is hard, but I appreciate that others will be capable of working it out and it's honestly hugely satisfying to complete a puzzle! It's such a masterful game.
I think you're working against some imbedded game concepts though, for example, generally you'd assume being that colour means you can only go through that colour wall or gate, so even though you've told the player that's not how it works, it's a case of re-wiring your brain to understand it (atleast for me).
Your concept is very clever, but it's introduced at a steep angle. Even the first level introduced me to more than one concept, which was:
1. A square can't move through a wall it shares a colour with.
2. The square can split if you do try to go through a wall it shares a colour with.
3. The squares can be mixed/merged back together again.
I like the level select though as I could complete level 3 but not 2, so it helped me continue playing. :)
Also from a colour theory perspective, it would help to know if these squares are light or paint. I can't remember for sure, but I think when lights mix, their primary counterparts mix to form white, but when paints mix, you can achieve black by mixing all the primary pigments. After analysing it, it's now obvious to me these are squares of light, but when I was playing, I didn't realise.
And the music is super catchy! (I'm still listening to it as I type this.) I think it matched the theme perfectly and it made the experience quite joyous even when I wanted to throw my computer out of the window because i'm not smart enough to work out the puzzles, haha!
Hope this was all helpful anyway. I really loved your submission, you've got a real talent for puzzle games - congratulations!
, so that's why I called the game Subtractive ♥