When I was studying psychology, one of my books gave a classic example of how a shade of gray appears lighter or darker based on the shade of surrounding colors. I had to fold the page over to convince myself that the gray blocks were indeed the same. The linked article is an example of this phenomenon using colors, and it’s pretty unbelievable at first glance. The blue and green spirals are actually the same color. Both cases are examples of the fact that our perceptions (how the brain interprets its inputs) can be radically different from reality.
Note: in the example with the shades of gray, I used an image editor to extract pieces of the two squares, and they are indeed the same exact color, even though one looks black and one looks white. I dragged a piece of the “black” square over to the shadowed area and it changed color before my eyes. Amazing.