It doesn't look like it, but the girl in the illustration above has two gray eyes. (And some vicious-looking fingernails.) This is due to the "opponent process" our brain uses to interpret signals from photoreceptors in our eyes--a process that sometimes produces weird and counterintuitive visual results. There are more illusions with full explanations in Scientific American's "Colors Out of Space" slideshow. Link

Image: Akiyoshi Kitaoka