An original idea! I very much enjoyed it.
My only feedback would be the first enemies that only have one colour, I would at least give them a black "outline" so they appear ghost-like when they go on a white background. Not being able to see what kills you at all is not so fun. They could, for example, have a blinking outline that blinks with a delay in between, so you can only see them occasionally when on a white background.
Otherwise a stellar game!