Josef Albers has explored some visual illusions due to colour contrast.
The first couple of images are taken from the cover of the paperback
edition of Albers book: Interaction of Colours.
The small brown square on the upper half seems to have a different
saturation from the lower one.
The second couple of images is another Albers' original example
of a smooth changing green pattern with a surrounding lighter grey cross:
the visual illusions (see left) makes the grey cross
to appear as non uniform.
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The right hand images confirm the illusion: you can check by testing the
HSV value.
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See:
Marini D. Rizzi A. "Colour Constancy and Optical Illusions:
a computer simulation with Retinex Theory"
, 7th ICIAP, Bari (1993)
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Marini D.,Marini L. & Rizzi A. "Towards a Computational Model of
Colour Perception",
Internal Report, DSI-Unimi (1995)
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Marini D., Marini L. "A computational model for quantification of colour
perception and its heuristic role in colour study"
, The Trieste Symposium
on Perception and Cognition, Trieste, October (1995)
