This one is especially hard to believe, at least for me, but it is indeed the same color. I did a cut and paste overlay to be sure, and it’s precisely the same. Amazing - and I already knew a good deal about the illusion effects when light and dark are interspersed… still got me though.
I don’t get it 
Is the green actually gray or something?
Square A is the same color as square B.
Click the picture above if you are in doubt, and you end up here:
http://antwrp.gsfc.nasa.gov/apod/ap070717.html
Nice one Denny, this is one of the better same color illusions I have seen.
/MarcusR
One thing to remember when dealing with a picture that Denny has posted…ALWAYS click on the picture before asking questions..all will be revealed 
And hey! HW, you’re almost at 100 posts! Getting up there, aren’t you? 
Denny’s messing with my mind! 
I had to stick little post-its to the screen to see it, but sure enough.
One of the art classes I took in college was “color.” Among the projects we did was creating small montages in which we were to make two different colors appear to be the same, and make two occurrences of the same color appear to be different.
There were varying degrees of success.
Denny, my eyes
Tell me what is wrong
Was I unwise
To leave them open for so long 
djm
(for Jackson Browne fans )
Yes, as humans, we can easily be deceived about a lot of things, and most of them we can only discuss in the rubber room. The checkerboard optical illusion of color differences demonstrates that we (our brains) make judgements about color by the colors that surround the color in question. The colored square, when surrounded by darker squares, will appear to be lighter in color than the same colored square surrounded by lighter-colored squares. Interior decorators need to be mindful of such phenomena when designing the color combinations for a room. For example, an upholstered chair that will look just right in a room with yellow walls may look simply awful in a room with green walls.
I made a short video clip that easily demonstrates that A and B are indeed the same color. Because I’m a nerd that way. 
I was following a link from the APOD site, and I happened upon
this video on youtube. It’s a neat CGI project playing around with
common graphical illusions.
That’s a great illusion. Illustrates the ‘active’ component in colour perception really well.
The perceived difference in shade between A and B is wholly a matter of brightness I think; chroma and saturation don’t seem to play a role, certainly chroma doesn’t. Does anybody know of examples of illusions like this which involve misperceptions of chroma?
That was very neat. I was going to use emmline’s method but I couldn’t find any post it notes, so this convinced me. It is still hard for me to believe they are the same, even though I know they are.
Cynth, click on the photo for the Astronomy Pic of the Day, and below the photo there is a link to verify that, indeed, the two squares that appear different in color are the same color. Their demonstration by connecting the two squares seems visually convincing to me.
The demo on the apod site didn’t work for me initially. I could clearly see a gradation of the grey tones in the solid line they drew between the A and B squares. It wasn’t until I blocked out the mitigating shadowing on either side that I could finally see that A and B were the same shade.
From the same apod page, there are a couple of links at the bottom:
The first takes you to a YerTube film that deals a bit with chroma phenomena: http://www.youtube.com/watch?v=w8Zz05t19dg
The second takes you to a site with examples and explanations of many illusions: Visual Phenomena & Optical Illusions I tried to get through the tour but was starting to get a headache.
djm
When I looked at that picture, Doug, it just looked like a gradually shaded color change from A to B. I just tried it again and changed the angle of my monitor screen and the explanation squares looked a little more the same, but not quite—although I believed intellectually that they were the same. Maybe my eyes are especially susceptible to the illusion or something. I actually had trouble interpreting Wanderer’s video even—I had to look at it quite a few times. There is something that bothers me about the darker squares in that one—the squares that are light for A are dark for B, well I guess that’s part of the illusion—but my mind seems to have a hard time dealing with what seems like a paradox I guess.
Been a while since your hundred post milestone hasn’t it Izz? Can you remember back that far?
Wanderer–I have tried to open your file on 4 different computers*:
My “old” iBook
A PC with XP
A slightly newer eMac
My daughter’s 1 year old MacBook.
No dice.
If daughter #1 were home I could try it on her Vista Dell laptop.
disclaimer:
who the h** has that many computers in one house?
Here’s the thing.
My laptop is sacred. It’s mine. (tell that to the 3 girls who I constantly have to chase off it.)
We have a PC because someone needed it for high school physics labs.
We have the eMac because 3 kids at a time could not do their homework on one computer.
Daughters #1 and #2 are in college, and each now have a laptop. Daughter #3, the Kid, and the Dad (who likes reading The Motley Fool) all wrangle over the PC and the eMac now.
on the XP
if you open Windows Media Player (or anywhere between the thingy in the left corner and the standard 3 thingys on the right)
right click on “Windows Media Player”
point at “File”
“Open URL”
paste the link & press enter
does it play?
Yes! I’m such a technorunt.
And it was so exciting, too.