four-color process: see printing.

color-field painting, abstract art movement that originated in the 1960s. Coming after the abstract expressionism of the 1950s, color-field painting represents a sharp change from the earlier movement. The production of the abstract expressionists involved a strong personal emotionalism, a painterly quality, and occasionally, as in the works of Willem de Kooning, elements of cubism. Color-field artists moved toward a more impersonal and austerely intellectual aesthetic. In their works they dealt with what they considered to be the fundamental formal elements of abstract painting: pure, unmodulated areas of color; flat, two-dimensional space; monumental scale; and the varying shape of the canvas itself. Painters associated with the movement include Ellsworth Kelly, Jules Olitski, Kenneth Noland, Frank Stella, and Morris Louis.

color index, in astronomy, difference in an object's brightness as recorded between any two well-defined bands of the electromagnetic spectrum by using optical filters of different colors. If blue and red filters are used, then the color index B-R would be zero for a white star (spectral class A0). The color index is positive for stars redder than a white star and negative for stars bluer than a white star. In effect, measuring the color index is equivalent to measuring the difference between the amount of blue light and red light that the star radiates. Although a star's true color might be changed as its light travels through clouds in interstellar space, the color index is an important indicator of the temperature of a star, gas cloud, or galaxy.

color blindness, visual defect resulting in the inability to distinguish colors. About 8% of men and 0.5% of women experience some difficulty in color perception. Color blindness is usually an inherited sex-linked characteristic, transmitted through, but recessive in, females. Acquired color blindness results from certain degenerative diseases of the eyes. Most of those with defective color vision are only partially color-blind to red and green, i.e., they have a limited ability to distinguish reddish and greenish shades. Those who are completely color-blind to red and green see both colors as a shade of yellow. Completely color-blind individuals can recognize only black, white, and shades of gray. Color blindness is usually not related to visual acuity; it is significant, therefore, only when persons who suffer from it seek employment in occupations where color recognition is important, such as airline pilots, railroad engineers, and others who must recognize red and green traffic signals. Tests for color blindness include identifying partially concealed figures or patterns from a mass of colored dots and matching skeins of wool or enameled chips of various colors.

color, effect produced on the eye and its associated nerves by light waves of different wavelength or frequency. Light transmitted from an object to the eye stimulates the different color cones of the retina, thus making possible perception of various colors in the object.

See also light; painting; protective coloration; vision.

The Visible Spectrum

Since the colors that compose sunlight or white light have different wavelengths, the speed at which they travel through a medium such as glass differs; red light, having the longest wavelength, travels more rapidly through glass than blue light, which has a shorter wavelength. Therefore, when white light passes through a glass prism, it is separated into a band of colors called a spectrum. The colors of the visible spectrum, called the elementary colors, are red, orange, yellow, green, blue, indigo, and violet (in that order).

Apparent Color of Objects

Color is a property of light that depends on wavelength. When light falls on an object, some of it is absorbed and some is reflected. The apparent color of an opaque object depends on the wavelength of the light that it reflects; e.g., a red object observed in daylight appears red because it reflects only the waves producing red light. The color of a transparent object is determined by the wavelength of the light transmitted by it. An opaque object that reflects all wavelengths appears white; one that absorbs all wavelengths appears black. Black and white are not generally considered true colors; black is said to result from the absence of color, and white from the presence of all colors mixed together.

Additive Colors

Colors whose beams of light in various combinations can produce any of the color sensations are called primary, or spectral, colors. The process of combining these colors is said to be "additive"; i.e., the sensations produced by different wavelengths of light are added together. The additive primaries are red, green, and blue-violet. White can be produced by combining all three primary colors. Any two colors whose light together produces white are called complementary colors, e.g., yellow and blue-violet, or red and blue-green.

Subtractive Colors

When pigments are mixed, the resulting sensations differ from those of the transmitted primary colors. The process in this case is "subtractive," since the pigments subtract or absorb some of the wavelengths of light. Magenta (red-violet), yellow, and cyan (blue-green) are called subtractive primaries, or primary pigments. A mixture of blue and yellow pigments yields green, the only color not absorbed by one pigment or the other. A mixture of the three primary pigments produces black.

Properties of Colors

The scientific description of color, or colorimetry, involves the specification of all relevant properties of a color either subjectively or objectively. The subjective description gives the hue, saturation, and lightness or brightness of a color. Hue refers to what is commonly called color, i.e., red, green, blue-green, orange, etc. Saturation refers to the richness of a hue as compared to a gray of the same brightness; in some color notation systems, saturation is also known as chroma. The brightness of a light source or the lightness of an opaque object is measured on a scale ranging from dim to bright for a source or from black to white for an opaque object (or from black to colorless for a transparent object). In some systems, brightness is called value. A subjective color notation system provides comparison samples of colors rated according to these three properties. In an objective system for color description, the corresponding properties are dominant wavelength, purity, and luminance. Much of the research in objective color description has been carried out in cooperation with the Commission Internationale de l'Eclairage (CIE), which has set standards for such measurements. In addition to the description of color according to these physical and psychological standards, a number of color-related physiological and psychological phenomena have been studied. These include color constancy under varying viewing conditions, color contrast, afterimages, and advancing and retreating colors.

Symbolic Uses of Color

Color has long been used to represent affiliations and loyalties (e.g., school or regimental colors) and as a symbol of various moods (e.g., red with rage) and qualities (e.g., worthy of a blue ribbon). A well-known use of the symbolism of color is in the liturgical colors of the Western Church, according to which the color of the vestments varies through the ecclesiastical calendar; e.g., purple (i.e., violet) is the color of Advent and Lent; white, of Easter; and red, of the feasts of the martyrs.

In topology, a long-standing conjecture asserting that no more than four colours are required to shade in any map such that each adjacent region is coloured differently. First posed in 1852 by Francis Guthrie, a British math student, it was solved by Kenneth Appel and Wolfgang Haken using a computer-assisted proof in 1976.

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Specialized printing technique using coloured inks and modified presses. Juxtaposition of colours is achieved by submitting each sheet to successive impressions by typeforms each of which prints only on areas designed to carry a single colour and inked only in that colour. Three colours of ink can reconstitute the visual effect of all the range of colours by combining them appropriately; if all three inks are applied to an area, it almost appears black. Standard colour printing, called four-colour printing, employs magenta, yellow, cyan (blue), and black inks.

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In igneous petrology, the sum of the volume percentages of the coloured, or dark, minerals in the rock. The most common light-coloured minerals are feldspars, feldspathoids, and silica or quartz; abundant dark-coloured minerals include olivine, pyroxene, amphibole, biotite, garnet, tourmaline, iron oxides, sulfides, and metals.

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Inability to distinguish one or more colours. The human retina contains three types of cone cells that absorb light in different parts of the spectrum. Absence of these types causes colour blindness to red, green, and blue. Colour blindness is a sex-linked recessive trait (see recessiveness) 20 times more common in men than in women.

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Aspect of any object that may be described in terms of hue, brightness, and saturation. It is associated with the visible wavelengths of electromagnetic radiation, which stimulate the sensor cells of the eye. Red light has the longest wavelengths, while blue has the shortest, with other colours such as orange, yellow, and green between. Hue refers to dominant wavelengths. Brightness refers to the intensity or degree of shading. Saturation pertains to purity, or the amount of white light mixed with a hue. The colours red, yellow, and blue, known as primary colours, can be combined in varying proportions to produce all other colours. Primary colours combined in equal proportions produce secondary colours. Two colours that combine to form white light are said to be complementary.

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