Neuroscientists usually explain color illusions in mechanistic terms: They arise because of the way cells in the retina and the brain respond to certain wavelengths of light. Those explanations miss the larger point, says Beau Lotto, a brain research at University College London. We misperceive colors and shapes because our visual sense has been molded by evolutionary history.
The blue dot on the top of the cube and the one on the darker side are exactly the same hue, reflecting light with identical intensity, or luminance. But because your visual system interprets the two-dimensional drawing as three-dimensional reality, you see the right side "in shade" and so perceive the right-side dot as bright, even glowing. In everyday life this perceptual bias is useful; it is what normally allows you to understand how distant objects occupy space.