<P> In normal human vision, wavelengths of between about 400 nm and 700 nm are represented by this incomplete circle, with the longer wavelengths equating to the red end of the spectrum . Complement colors are located directly opposite each other on this wheel . These complement colors are not identical to colors in pigment mixing (such as are used in paint), but when lights are additively mixed in the correct proportions appear as a neutral grey or white . </P> <P> For example: the reason that the Wimbledon tennis tournament uses purple on the Wimbledon official logo is that purple is located almost opposite of green on the color wheel . Purple against green provides good contrast . </P> <P> The color circle is used for, among other purposes, illustrating additive color mixture . Combining two colored lights from different parts of the spectrum may produce a third color that appears like a light from another part of the spectrum, even though dissimilar wavelengths are involved . This type of color matching is known as metameric matching . Thus a combination of green and red light might produce a color close to yellow in apparent hue . The newly formed color lies between the two original colors on the color circle, but they are usually represented as being joined by a straight line on the circle, the location of the new color closer to the (white) centre of the circle indicating that the resulting hue is less saturated (i.e., paler) than either of the two source colors . The combination of any two colors in this way are always less saturated than the two pure spectral colors individually . </P> <P> Objects may be viewed under a variety of different lighting conditions . The human visual system is able to adapt to these differences by chromatic adaptation . This aspect of the visual system is relatively easy to mislead, and optical illusions relating to color are therefore a common phenomenon . The color circle is a useful tool for examining these illusions . </P>

What kind of color harmony was used in this image