This is the incredible Munker - White illusion. The truth about what you are seeing is that the blue, red and grey lines in each rectangle are actually the same color, not a light or dark version. The reason for this massive illusion is not well understood by scientists, but it is so powerful that it is hard to believe, unless you do like I did and cut a small slot in a piece of paper and do a comparison while blocking out the rest of the image. Lo and behold, they are the same color!

Fuh-ree-kee!