[Excerpt from Cornell.edu]

Color Illusions
Some of the effects of color occur only in the eye and brain of the viewer, and are not physical properties of light waves or pigment. These illusions, however, are very powerful, and have enormous impact on our responses to color.

Color Proportion refers to the impact of the relative quantity of a given hue or value used in color compositions. In order to achieve over-all unity, and/or create emphasis, one should make a clear decision as to which colors should be assigned the largest and least areas. The color proportion choice will also affect the impact of the color composition. This can be seen in the set of panels shown here. The very same colors are used in each panel. Yet depending on the choice of dominant color, the feeling of the composition, and even the appearance of each color, is altered.

Simultaneous Contrast is the phenomenon which occurs when a color appears to change when seen against a different background. A set of principles were first laid out in the 19th century by Chevreul, a dye master for the Gobelin tapestry works, who became an important color theoretician. His principles state that changes in the hue, value, saturation (purity of hue), and area of a background color will alter the appearance of the selected color. The print shown here is made up of wavy bands of colors. Some of the bands extend from the center panel to intrude into areas of contrasting hue in the side panels. These extended bands are in fact the same hue and value throughout, but appear to change from left to right.

If you are interested in further information about how our visual response to color may vary, see this section on optical effects in color.

Optical mixture is the phenomenon which occurs when small particles of different colors are mixed in the eye; this type of mixture differs from pigment mixture in that it is based on light primaries. However,optical mixture differs from light mixture in which the primaries will mix to white, and from pigment mixture, in which the primaries mix to black. In optical mixture there is an averaging of hue and value, resulting in grey. Optical mixture is experienced when observing many textiles, such as this example, a detail from a handwoven tapestry. It can also be seen in natural objects, color television, and printed color pictures.

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For a more in-depth look at the psychological implications of color, check out any of the following:

• Effects of color on how we think and feel.
• Effects of color on food and appetite
• Effects of color on the body.

Other websites that offer general information on color:
[…A] page of optical color effects
A good overview of color systems for designers