olour constancy is a name given to the phenomenon that the eye and brain perceive something to be a consistent colour under different lighting conditions. Sometimes this effect can be quite marked, see for example the illusions shown on echalk.co.uk where a neutral grey can look blue under one simulated lighting condition and yellow under another.

See also my page Colour Constancy Examples.

The eye is not a camera. What you are seeing is not a direct interpretation of the light beams that fall on your retina, your vision system is making assumptions and hypotheses all the time. Scientific backup for this assertion can be found especially in the work of the neuroscientist David H. Hubel, see David Hubel : The Brain and Visual Perception on YouTube and his book, Eye, Brain and Vision (1988, revised 1995) published by Scientific American Library.

I am interested in something slightly different from the demonstrations of lighting conditions, more my interest is in how colours are being perceived in relation to what is next to them. There has been a lot of work done on this phenomenon using coloured squares, though not much, so far as I know, in real-world conditions. Much of my interactive software on these pages is designed with such phenomena in mind, as are many of the artworks you can reach from the icons at the head of this page.