Have a look at this spiral of colors above. Do you see the green and blue spirals going toward the center? Well, what you see as green and blue is actually the same exact color (Discover Magazine has RGB proof). The visual trick here occurs because our brains “see” colors based upon the colors that surround it. In this case the spiral is separated by magenta and orange stripes and the magenta stripes don’t actually go into the green spiral and the orange stripes don’t actually go into the “blue” spiral. It is because of the juxtaposing colors that our brain gets a bit confused as to what color is truly there.

If you don’t see it in the image above, Richard Wiseman has posted an alternate version of the same optical illusion where you can better differentiate the magenta, orange, and green lines (note that the blue lines here are actually the same color as the green ones).