Definitely a very well done difficulty curve for the majority of the game. Then the last 2 levels come in and just hit you with all the stuff, once I figured out the base mechanics... and... thing, for the last one, it was pretty fun to puzzle through how to actually complete it.
Otherwise though a couple points (that I mostly mentioned) would be:
- Some sort of undo button could be nice, though I guess it could also trivialize testing every color combination on certain levels so maybe just short levels (like what there is now) with some sort of slightly faster movement? That way it ignores the problems of an undo, but also helps solve what the undo should.
- A few of the colors were, as I think has been said, a little too close for me personally but it was still a relatively clear indicator of what I was doing and where.