Thank you ! You are right an undo feature would be good.
Actually, I tried to take the theme into account in a very subtle way. The idea was to force the player to move even if he had completed the puzzle. As the following image shows:
The puzzle may be finished, but the game will not consider it finished because the player still has red paint in stock. So, the player is forced to move and potentially fail the puzzle. The idea was that the player could not stop moving as soon as he finished the puzzle (the main goal of the game), but only when he ran out of paint. I hope you will understand how I take the theme into account