HOWEVER, in a sense it's a semantical illusion rather than a purely optical one, and it "cheats" by using illumination angles that would, in life, make the top section necessarily darker than the lower section. "techically", it's the same color, but in a sense that isn't really relevant other than being an interesting exercise in perception and interpretation of how we expect things to behave assuming they're subject to the physics of the real world.
This is the singularly most awesome response to this illusion I've ever seen.
As well as the follow up.
In other words, if the same shade of gray is used to represent the illumination of a darker object and the shadow of a lighter object, are those portions of the object really the same color?
If the object exists, then no. Only the color used to represent the two parts of the object are the same.
Nigel, I'd really love to invite you over for dinner parties at my place.