How do i script a check on drawing canvas if all the pixels in it are white or not?

I am making a coloring game and i want to promote the player to next sketch as soon as the player leaves no white color on the canvas (drawing canvas).

P.S. I have already filled the drawing canvas white before player starts coloring it.

You need to write a function that returns True/False depending on whether there is at least 1 non-white pixel on the canvas. To do this, this template will help you.