it's a clever idea but the perception of the color is not easy to have a graduated difficulty. Plus, an alternative to the div might be to use the canvas to avoid cheating by watching the position of the gradient on the console :p

anyway, thank you ! i didn't clean my screen since 1 week, and this game need to !

0.000020 ^^ https://www.keithcirkel.co.uk/whats-my-jnd/?r=AAIoKP