How does the eye perceive colour? Probably a little differently than you thought. Context is everything, as the following link shows:

http://www.echalk.co.uk/amusements/OpticalIllusions/colourPerception/colourPerception.html

This may no longer be available from September 2005, so I’ve taken the liberty of reproducing it here:

The centre section of each structure is exactly the same colour, even though one looks grey and one looks yellow. If you don’t believe me, copy the image to your favourite paint program and check out the RGB values.