http://www.patmedia.net/marklevinson/cool/cool_illusion.html

wtf lol

Yet another reason why I don't trust my body to tell me whats up from down. Sheesh.

Now why does that happen!?

I have no idea, but when the "green" dot circles around erasing the others, I can hear Jesus whispering in my ear.

If you take the .gif and watch it 1 frame at a time there are a few clues.

The actual image:
http://www.chadhorn.net/misc/image.gif

The image freezed 1 frame:
http://www.chadhorn.net/misc/image.jpg

Closeup:
http://www.chadhorn.net/misc/image2.jpg

The green is always there, its just "hidden". I think its part of the persistance of vision, where you eye retains an image for a split second after you see it. When you watch an animation, you only see a series of probably 16 or 24 or 30 images, but your mind blurs them together and creates whats in between. Your mind is simply blurring out the green thats actually there unless you focus on the static cross.

There isn't any green in those images. The "hidden" part is #B2B2B2.

I don't think that's how it works.

its the gnomes, and grimlings I tell you....

hrm so when its bordered by the purple it looks greenish. Weird.

Partial explanation - color as light is cool stuff: http://www.null-hypothesis.co.uk/science/strange-but-true/profs-probings/colour_spectrum_magenta_complimentary_bizarre