I just stumbled upon what might be the greatest optical illusion I've ever seen.
Take a look at this image:
Do you see blue and green? I did too, then I was told they're the exact same colour. I have a colour picker add-on for Firefox that lets me click a colour to see the RGB numbers and lo and behold both the blue and green are rgb(0, 255, 150).
The reason they look different colors is because our brain judges the color of an object by comparing it to surrounding colors. In this case, the stripes are not continuous as they appear at first glance. The orange stripes don’t go through the "blue" spiral, and the magenta ones don’t go through the "green" one.