I've had a go at your game and finished it!
Scholley is right about the jumping, its a bit too fast. And remember what orders the colors were in was a problem for me as well.
The grass that kills you if your the wrong color felt a bit brutal, not that the idea is bad, just that it seemed like even if the tip of my slime was over it when i changed color i died. Should be an easy adjustment by just resizing the hitbox I would think.
As far as the boss goes, I think it should have a more predictable color change pattern.
I beat it, but i felt like in the final hp point, where he flickers the fastest, I got through it from luck rather than skill. Also I think it would be cool if the boss stage required some kind of jumping as well.
I don't mean to be too negative however, as the idea is quite interesting.
The graphics, sound effects and music are all quite good!
Good luck with further developing your game! :)