You can prove it just by following the echalk link, or by making a small hole in a piece of paper to mask out the context. Then you see that the colours really are the same.
http://www.echalk.co.uk/amusements/O...erception.html
It's offline at the moment, so here's another link to something Arkain posted re the first image. He didn't quite crop them right, but you can see that the central portions are both gray.
http://hypography.com/forums/142242-post40.html
