Colors Out of Space

It doesn't look like it, but the girl in the illustration above has two gray eyes. (And some vicious-looking fingernails.) This is due to the "opponent process" our brain uses to interpret signals from photoreceptors in our eyes--a process that sometimes produces weird and counterintuitive visual results. There are more illusions with full explanations in Scientific American's "Colors Out of Space" slideshow. Link

Image: Akiyoshi Kitaoka

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So here is a great article from "Scholarpedia: The Peer-Reviewed Open Access Encyclopedia" this article on color-vision including the opponency process is written by Karen K. DeValois and Michael A. Webster.
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A very common kind of color-based illusion. If you find something wrong with this one, there are thousands more.

For a complete explanation pick up one of these books (they all explain it): The Engine of Reason The Seat of the Soul by Paul Churchland, Brain-Wise by Pat Churchland, The Astonishing Hypothesis: The Scientific Search for the Soul by Francis Crick, The Quest for Consciousness: A Neurobiological Approach by Christoph Koch.

There is also a video called The Neuroscience of Nothing which explains contrast luminosity, but the simplest way of explaining any of this kind of thing is to point to the completely relative nature of everything that the human mind produces. Color is not a concrete empiric quality, but is a relative continuum generated by your mind. Just like... your idea of your self and your idea of an actually existing objective world.
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@Seban: according to the dictionary, an illusion is "something that deceives by producing a false or misleading impression of reality". In my watering can example, reality is the real color of the watering can. And since the real color of the watering can is blue, there would be no illusion -- there's no deceiving going on when something blue is being perceived as blue.

An optical illusion is when you're deceived into thinking that two lines have different lengths when in fact they have the same length; or when you're deceived into thinking something is moving when in reality it's static -- but when you're being "deceived" into thinking that something is blue when in reality it actually is blue... not much of an illusion.

Also, the eye is indeed a bit greenish, which is indicative of the fact that whoever created this image probably did exactly what I described in my watering can example above: I guess they actually painted the eye blue, and then fiddled with a red filter until they reached the point where the filter approximated grey for the eye. If the "illusion" had been constructed cleanly then there would've been no color spill (as proper, geometrical illusions are constructed, like the other three in the gallery mentioned in the post).
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Feodor, you don't seem to have grasped the concept of optical illusions. The left eye in the picture "actually" (as in, the pixels that are displayed by your computer screen) is grey. Your brain then interprets the picture as having a filter placed over it, and therefore "sees" the eye as being blue.

So.. pixels -> grey; what you see -> blue. Hence: optical illusion.

By the way, to all the left-eye-greyness-deniers, the left eye is grey. It just is, you know it, everybody who can use MS paint knows it, and trying to find bluish pixels is missing the point.
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