The Land-Mondrian Experiment
If a multicoloured scene is illuminated with three projectors (this is known as the NATURAL condition), the brain compares the light coming from all the patches in the scene in order to assign colours to the patches. If you illuminate only one patch, leaving the others dark (known as the VOID condition), then the result of the comparison (between the patch and nothing else) will be different, leading to a different perception of the colour of the patch. To convince yourself that the actual emission of the patch is remaining constant between the two conditions, cut a hole in a piece of black card and hold it over the screen whilst you press the buttons.
For more information see: 'A demonstration of 'void' colours and 'natural' colours', S. Zeki, Journal of Physiology 341, 1 (1983)
Move your cursor over the image to toggle it between the natural and void states