>Our perception of colour is strongly affected by the surrounding colours.
From a [pretty neat demonstration page](http://www.echalk.co.uk/amusements/OpticalIllusions/colourPerception/colourPerception.html) (via [Lex](http://www.thefriedmans.net//blog/comments/its_all_an_illusion)).
This is so interesting me. Color is so much about expectation and relationships. Light is reflected in a certain range and our brain interprets that range to match information that we know by comparison. We know “green” because it’s the grass, the trees, a granny smith apple. We know it’s green because we have a point of reference. But how do I really know that what I’m seeing as “green” is what everyone else is seeing? It’s green because in my childhood, items that reflected light to my eye in this range were identified to me as “green” and I saw the consistency between those items and knew that anything that looked like this was henceforth “green.”
So in illusion 3, no matter how hard I try to convince my brain that it’s seeing two gray joints, my eyes are still sending the signal to my brain that what I see is yellow. For all I know what I see as yellow is magenta to someone else, but yellow is its name.