Color, defined generally, is the aspect of any object that may be described in terms of hue, lightness, and saturation. In physics, color is associated specifically with electromagnetic radiation of a certain range of wavelengths visible to the human eye. Radiation of such wavelengths comprises that portion of the electromagnetic spectrum known as the visible spectrum--i.e., light.

Major regions of the visible spectrum are seen as different colors by the eye, so that the spectrum appears as a band of colors ranging from red at the long wave end, through orange, yellow, green, blue, and indigo, to violet at the short wave end. Sir Isaac Newton investigated this sequence of colors in a series of experiments on white light in the mid-1660s and made a classification of seven elemental colors in a prototype of the modern color wheel.

Three characteristics that are commonly used to distinguish one color from another are hue, saturation, and brightness. Hue is an attribute associated with each of the dominant wavelengths of the spectrum. Saturation pertains to relative purity, or the amount of white light mixed with a hue. High-saturation colors contain little or no white light. Brightness refers to intensity, distinguished by the degree of shading. Hue and saturation, taken together, are called chromaticity. Accordingly, a color may be characterized by its chromaticity and brightness.

Many ways of specifying and classifying color have been devised since Newton's invention of the color wheel. Among the most significant is the Munsell color system developed in 1913 by Albert H. Munsell of the United States. This system defines and arranges colors on the basis of hue, value, and chroma, characteristics that correspond respectively to dominant wavelength, brightness, and purity. Another widely consulted color system was devised a few years later by Wilhelm Ostwald, a German chemist. Ostwald's method designates colors in terms of purity, whiteness, and blackness. The most widely used system of colorimetry is that adopted in 1931 by the Commission Internationale de l'clairage (CIE), commonly called the International Commission on Illumination. The CIE system, revised in 1964, employs three values that correspond to red, yellow, and blue as basic reference points, designating them as the three primary colors from which all other colors are derived. Any desired hue can be produced when various amounts of two of the primaries are mechanically combined either by addition or by subtraction. Addition involves mixing parts of the spectrum, whereas subtraction entails the removal or absorption of spectral components.

Colors are said to be complementary if they form white when they are combined additively or black when mixed subtractively. It is apparent that the complement of any color is the mixture of the other two in the triad.

Excerpt from the Encyclopedia Britannica without permission.