I looked at squares A and B and found that if I stared at it for a few seconds, I could see both squares as being the same color. (Of course, the best way to see it would have been as a checkerboard pattern multiplied by a shadow with a maximum ratio equal to the light/dark ratio; I definitely couldn't do that.) I decided to check whether I could also see a light square outside the shadow and a dark square inside the shadow as being the same color, but the time I decided that, I noticed that I could no longer see A and B as being the same color. I'll have to have another go at it in a few minutes.

(Which is now, as this post took me a couple of hours to write.)