Move your mouse around to create varying size squares of color on the background. A few built in options:

"Toggle" - turns on a mode that makes squares toggle on/off as you mouse over them.
"Random Color" - makes squares colored randomly (opposed to red)
"Square Dimensions" - lets you change the size of the squares. must click Reset to take effect.
"Erase" vs "Paint" mode - instead of painting colors onto a bg, you can instead start with squares over and image and erase them to reveal the background

Note that once a square has been painted it won't change color unless you set toggle mode on.

View another demo where you erase away tiles to reveal an image.
Download the js file here