A mixture of three pure tones (top) yields a complex resultant tone (bottom), such as might be elipsis
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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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Opaque watercolour. Also known as poster paint, designer's colour, and body colour, it differs from transparent watercolour in that the pigments are bound by liquid glue, which is used as a thinner. The addition of white pigment lightens the tone and lends opacity. Gouache paints dry to a matte finish and, if desired, without visible brush marks. They can be applied thinly or thickly. A wide range of colours are available, including fluorescent and metallic pigments. The suede finish and crisp lines characteristic of many Indian and Islamic miniature paintings is produced by this medium; it is used in Western screen and fan decoration and was used by modern artists such as Georges Rouault and Paul Klee.
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