Lots and Lots and Lots of Illusions by AJC
Believe it, there is no green in this picture!
When you look at gray, all three types of cones work in your eye. However, when looking at really bright colors, your cones respond differently depending on the wavelengths coming from the color. In this illusion, we use the red and blue cones to make the color magenta.
If we stare at the cross in the middle of this picture, an area of our eyes stop working for a moment. This leaves just the green cones in our eyes working, so that is why we see green. Scroll back up and you will see this all starts to make sense!