Neon Color Spreading
Neon color spreading is a visual illusion in which an illusory shape (as defined by apparent edges that are not physically present) appears to take on a color. For example, when looking at the figure below most people will perceive a teal colored circle in the center. The only place where the teal color physically appears is on the arcs. The circle itself is not colored. teal. The fact that the circle appears teal is illusory -- all the space between the arcs is white.
Neon color spreading should remind you of illusory contours -- the colored circle is defined by a contour that does not physically exist. However, neon color spreading is not identical to illusory contours as one will occur under certain lighting conditions while the other will appear under different conditions. Watanabe and Sato (1989) manipulated the luminance of the figure so that it was either lower than, equal to, or greater than the luminance of the background. When the figure and background were isoluminant (had the same brightness), the illusory contours disappeared but the color spreading remained.
Several theories have been put forth to explain neon color spreading, but most have problems that they cannot easily address.
You can change each of the arc colors and the background color by clicking on the appropriate color control. Do some combinations of colors produce more vivid neon color spreading than others? Can you come up with a hypothesis as to which set of colors produce more vivid neon color spreading and which produce less vivid neon color spreading?
When forming hypotheses or changing the colors, it is sometimes better to think about color in terms of its psychological dimensions -- hue, saturation and brightness (luminance). The color picker control on some browsers (e.g. Chrome and Firefox) allows you to change hue, saturation and brightness directly. Other browsers (IE11) do not. You are strongly encouraged to use a browser that allows manipulation of the psychological properties.
Watanabe, T., & Sato, T. (1989). Effects of luminance contrast on color spreading and illusory contour in the neon color spreading effect. Perception & Psychophysics, 45, 427-430. http://dx.doi.org/10.3758/BF03210716