What happens when you stare at this oddly colored arc? Your brain will make a candy-colored surprise out of thin air.
Why do you see a rainbow where there is none? This type of illusion is called an "afterimage" illusion. There are two kinds of afterimages - negative and positive. In a positive afterimage, the original color of the image is retained, but in a negative afterimage, like this rainbow illusion, the colors become inverted.
Negative afterimages happen because staring at something brightly colored overtaxes the visual system's cells and they become overstimulated, which makes them less sensitive. In response, the cells for the bright color weaken while their opposing color signal strengthens. In other words, staring at something green for too long will result in a reddish afterimage.