Blue-barred

This illusion demonstrates the tendency of the brain to process colour information in a relative manner.

The blue stripes are of uniform hue and lightness, yet they seem darker when surrounded by black stripes and lighter when surrounded by white stripes.
Extensive scholarly research and debate have failed to come up with an accepted explanation.

'There has been a long debate on the different mechanisms underlying the "Munker-White Illusion" says a recent study. The study effectively shows that each of the explanations given to date is insufficient to explain the very strong illusion. The illusion therefore seems to be the result of a complex interaction of several factors such as Lightness Constancy (see Misleading Shadow illusion), but not including lateral inhibition (see Lightness is Relative), as this would give the opposite effect!

Links:
For the illusion itself see:
http://www.michaelbach.de/ot/lum_white-illusion/index.html

For a detailed discussion of possible explanations see BartPerception2001.pdf