I really like the puzzle design in this game and how each level teaches you new mechanics one by one. I don't have any "things to fix" feedback for the game that has not been said by others, so I'd like to give some ideas that could be cool to add in an update.
I think the game would benefit greatly from a more detailed pixel art style, like adding some brick walls texture and a little bit of shading to add to the atmosphere. I feel something similar to the pixel art in Otherworld Legends would fit the aesthetics of the game nicely. There could also be rooms with themes that highlight certain emotional concepts.
There could also be enemies that patrol an area as you move around and a flame that mix the next color you pass through with the color you have now. These would greatly increase the puzzle complexity and variety.
Of course, these are just suggestions but hopefully they will help if you want to develop this game in full production. Will be looking forward to it if you do :)