check this out
http://web.mit.edu/persci/gaz/main-frameset.html
http://web.mit.edu/persci/people/adelson/checkershadow_illusion.html
Yes it is quite wierd isn't it? I saw a show on TV about it recently using colours not just grey, and depending on the surrounding colour, the colour appeared to change dramatically. Very odd.
No quite the same thing but... A painter freind of mine had mixed a paint colour for a lady and painted part of a wall for her to check if it was what she wanted. She insisted it was nothing like what she had chosen. >:( He went away and came back the next day with the same paint but this time he painted the test area on a different wall. "Yes that is exactly what I chose" was her response!! ;D
Allan