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Abney phenomenon: “change of hue produced by decreasing the purity of a colour stimulus while keeping its dominant wavelength and luminance constant” (CIE, 2011, 17-1).

Accuracy: in drawing and painting, the relative extent to which the visual appearance of a work resembles the appearance of the subject considered as a two dimensional projection onto the picture plane.

Achromatic colour: colour lacking hue, as in white light, white, grey, black diffusely-reflecting objects, and colourless transparent and metallic objects. See also neutral.

Adaptation: any process by which perception is influenced by previous and present exposure to stimuli, for example the brightness (brightness adaptation) and the chromatic qualities (chromatic adaptation) of the illuminant.

Additive mixture:  “stimulation that combines on the retina the actions of various colour stimuli in such a manner that they cannot be perceived individually” (CIE, 2011, 17-24). Examples include mixing of superimposing beams of coloured light, and of light from the RGB subpixels on a screen.

Additive-averaging mixture:  “a form of color-stimulus synthesis in which the result is an average of the components rather than the sum” as in simple additive mixing (Burnham et al., 1963, as averaging color-stimulus synthesis). Examples include spinning disc mixture.

Afterimage: visual perception persisting after cessation of the stimulus, including positive and negative afterimages.

Analogous colours: in traditional "colour harmony", colours similar to each other in hue.

Assimilation: convergence of perceived colour observed when small areas of colour are interspersed.

Atmospheric perspective: progressive change in appearance of objects with increasing distance, due to increasing thickness of air intervening between the object and the observer.

Bezold-Brücke phenomenon: “change of hue produced by changing the luminance (within the range of photopic vision) of a colour stimulus while keeping its chromaticity constant” (CIE, 2011, 17-86). For example, single wavelengths of the spectrum shift in hue towards either blue (below 500 nm) or yellow (above 500 nm) as they become brighter.

Blackness: degree to which an object colour is judged to approach black.

Brightness: 1. “attribute of a visual perception according to which an area appears to emit, or reflect, more or less light. NOTE The use of this term is not restricted to primary light sources” (CIE, 2011, 17-111). Brightness is our perception of physical luminance, as conditioned by the brightness adaptation of the observer and by contrast phenomena and assimilation. 2. With respect to object colours, commonly used for brightness relative to the maximum possible for a given hue and saturation, for example brightness “B” in HSB colour space. White, the maximum-chroma colour and all tints in between are all "bright" in this sense.

Brilliance:  a scale of colour appearance along which related colours pass with increasing brightness from the black threshold through decreasing degrees of black content through a point of zero blackness (or “zero greyness”) to a fluorent (fluorescent looking) and ultimately self-luminous appearance.

Broken colour: 1. paint application using visible colour variants, as opposed to solid colour. 2. partly neutralized paint colour.

“Chalky” colour: colour that appears too whitish in its context in a painting.

Chroma: “colourfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting” (CIE, 2011, 17-139). Chroma is the departure of a colour from a grey of the same lightness, and is our perception of an object’s efficiency as a spectrally selective reflector/transmitter of light. Commonly confused with saturation.

Chromatic colour: colour having a hue.

Chromaticity: “property of a colour stimulus defined by its chromaticity coordinates, or by its dominant or complementary wavelength and purity taken together” (CIE, 2011, 17-1444). Chromaticity describes the psychophysical colour of a light, independent of its intensity (luminance).  Commonly misused as meaning chroma or colourfulness.

Chromaticness: In NCS terminology, perceived proportion of pure chromatic colour considered as a component of an object colour.

Chromostereopsis: perception of depth evoked by differently coloured areas on the same plane. Most observers see red objects “advance” and blue object “recede”, although a minority see the reverse.

CIE (Commission Internationale de l' Eclairage):  international organization responsible for recommendations for photometry and colorimetry.

CIE L*a*b* colour space: three-dimensional, approximately uniform colour space produced by plotting in rectangular coordinates L* (CIE lightness) and a* and b* (chromatic coordinates corresponding roughly to redishness/greenishness and yellowishness/bluishness respectively). A version of CIE L*a*b* called Lab space is of central importance in Photoshop.

CIE L*u*v* colour space: three-dimensional, approximately uniform colour space produced by plotting in rectangular coordinates L* (CIE lightness) and u* and v* (chromatic coordinates corresponding roughly to redishness/greenishness and yellowishness/bluishness respectively).

CIE xyY colour space: colour space produced by combining the dimension of luminance (Y) with the CIE xy chromaticity diagram.

Colour: 1. perceived colour: “characteristic of visual perception that can be described by attributes of hue, brightness (or lightness) and colourfulness (or saturation or chroma)” (CIE, 2011, 17-198) 2. psychophysical colour: “specification of a colour stimulus in terms of operationally defined values, such as 3 tristimulus values” (CIE, 2011, 17-197). Note that the same psychophysical colour, for example specified by a set of RGB values, can appear as different perceived colours when placed in different contexts. 3. (Loosely) a coloured pigment, paint, etc. See also luminous colour, nonluminous colour, object colour.

Colour constancy: the tendency of perceived object colours to remain constant when the illumination is changed.

Colour harmony: relationship of colours deemed to be aesthetically pleasing, often codified in traditional colour theory into rules concerned with hue relationships.

“Colour mixing”: loose term for additive, additive-averaging and subtractive light/colourant mixture, reflecting the naïve perception that colours reside and mix in the lights/colourants themselves.

Colour space: geometric representation of colour in space, usually of 3 dimensions (CIE, 2011, 17-226).

Colour string: a sequence of colour mixtures on a palette, for example of increasing white content, or of successive value steps for a given hue and chroma.

Colour symbolism: most colours have a wide range of often contradictory associations, generally both positive and negative, even within a single culture.

Colour temperature: 1. in lighting, “temperature of a Planckian radiator whose radiation has the same chromaticity as that of a given stimulus” (CIE 2011, 17-231). In this sense, reddish and yellowish lights have low temperatures and bluish lights have high temperatures. 2. in traditional colour theory, separation of colours into warm and cool colours (q.v) based on psychological associations. Reddish and/or yellowish colours are generally considered warm and bluish + greenish and greysh colours are considered cool, but usage is inconsistent between different strands of tradional colour theory.

Colour wheel: term used in traditional colour theory for a circular diagram of hues.

Colourant:  material including dyes and pigments used to impart chromatic colour to another material.

Colourfulness: “attribute of a visual perception according to which the perceived colour of an area appears to be more or less chromatic” (CIE 17-233).

Complementary colour: the opposite or “completing” colour. Originally applied to colours of light that mix to make white light (additive complementaries), but subsequently extended to include subtractive, paint-mixing and afterimage complementaries.

Complementary, additive: Hues of two lights that mix to make white light.

Complementary, afterimage: Hue of the negative afterimage of a colour.

Complementary, paint-mixing: Hues of two paints that physically combine to make a neutral mixture.

Complementary, split: In traditional colour theory, combination of a colour with two near complementaries, often the two colours adjacent to the complementary on a 12-hue “colour wheel”.

Cool colours: colours deemed to have a psychological association with coldness (see colour temperature). In various accounts the coolest hue ranges from violet through blue to blue-green.

Cyan: 1. the greenish blue hue of process cyan inks, generally based on the pigment phthalocyanine blue. 2. as a digital colour, the distinctly more greenish blue-green hue produced when the B and G components are equal.

Divisionism: painting style in which colour is “broken” into dots etc. of saturated or “spectral” components.

Dominant wavelength: any mixture of wavelengths can be matched in colour by a mixture of white light and a light of its dominant wavelength.

Dye: soluble colourant (as opposed to insoluble colourants, called pigments).

Edge spectra: spectra produced by progressively removing wavelengths of light beginning at the short- (white-yellow-orange-red) or the long- (white-cyan-blue-violet) wavelength end.

Fluorent: having the appearance of physical fluorescence.

Fluorescence: the physical process by which a material absorbs ultraviolet radiation and emits its energy as light, making it appear anomalously bright.

Gamut: range of colours obtainable by mixing a given set of colourants or lights.

Glaze: a transparent layer of paint used to modify the colour of an area.

Grisaille: painting or paint layer using only shades of grey.

Halation: appearance of light spreading out, for example from a light source viewed in dark surroundings.

Half light: zone of light moderately to strongly inclined to the light source, between the full light and the terminator.

Helmholtz-Kohlrausch effect: tendency of high-chroma colours to create an impression of greater brightness than a grey of the same luminance.

Highlight: specular reflection of a light source.

HLS: digital colour space classifying RGB colours according to hue angle (H), and rather arbitrary dimensions called “lightness” (L) and “saturation” (S) but unrelated to the standard definitions of those terms.

HSB: digital colour space classifying RGB colours according to hue angle (H), relative saturation (S) and relative brightness (B).

Hue: “attribute of a visual perception according to which an area appears to be similar to one of the colours: red, yellow, green, and blue, or to a combination of adjacent pairs of these colours considered in a closed ring” (CIE, 2011 17-542). Hue is expressed by position around a hue circle or “colour wheel”, and is our perception of spectral imbalance relative to a white light or object.

Hue angle: specification of hue in some colour spaces including the digital colour spaces HSB and HLS.

Hue page: array of colours of a single hue, arranged according to colour attributes such as lightness and chroma.

Hue shift: in painting, tendency of a paint to change visibly in hue as a result of thinning or the the addition of white, black or grey paint etc.

Hues, spectral: hues evoked by single wavelengths, in the sequence spectral red, orange, yellow, green, blue, violet.

Hues, non-spectral: hues evoked only by mixtures from the two ends of the spectrum, comprising purplish colours including magenta and middle red.

Hues, unique: “hue that cannot be further described by the use of hue names other than its own. … There are 4 unique hues: red, green, yellow and blue forming 2 pairs of opponent hues: red and green, yellow and blue” (CIE, 2011, 17-1373). Also known as opponent hues, unitary hues, or psychological primaries.

Illuminant: light source. The CIE has defined various standard Illuminants (e.g. D55) by their relative spectral power distributions for use in colourimetry.

Isoluminance: equal in luminance. A pattern of two or more isoluminant but otherwise strongly contrasting colour fields is visually disturbing

Light: electromagnetic radiation able to excite the human visual system. (The expression “visible light” is redundant).

Lightfastness:  resistance to colour change on exposure to light. Blue Wool Scale

Lightness: brightness of an area judged relative to the brightness of a similarly illuminated area that appears to be white or highly transmitting (CIE 2011, 17-680). Closely similar in meaning to value.

Light-shedding illusion: optical illusion that creates the appearance of luminosity or a luminous mist.

Luminance: visible energy of light, that is, physical light energy weighted according to the wavelength-by-wavelength response of the human visual system.

Luminous colour: “colour perceived to belong to an area that appears to be emitting light as a primary light source, or that appears to be specularly reflecting such light” (CIE, 2011, 17-727).

Lustre: appearance of a material related to the way light interacts with its surface. Terms used to describe lustre include glossy, matte (dull), vitreous (glassy), adamantine (sparkling), metallic, greasy, waxy and silky.

Masstone: colour of a paint when applied thickly (as opposed to undertone).

Metameric colour stimuli: “spectrally different colour stimuli that have the same tristimulus values in a specified colorimetric system” (CIE, 2011, 17-768).

Metameric failure: breakdown of colour matching between physically different surfaces under different illuminations.

Monochromatic: having a single hue. Commonly used for light of a single wavelength, and for a set of object colours having the same hue but varying in lightness and chroma.

“Muddy” colour: colour that appears too blackish in its context in a painting.

Munsell system: classification of object colours in terms of specific scales of hue, value (=lightness) and chroma.

Neutral: 1 achromatic 2. close to achromatic, to a greater or lesser degree.

Non-luminous colour: colour perceived to belong to an area that appears to be transmitting or diffusely reflecting light as a secondary light source (CIE, 2011, 17-818)

Object colour: “colour perceived as belonging to an object” (CIE, 2011, 17-831).

Opponent hues: see Hues, unique.

Optical mixing: appearance of two or more coloured stimuli as a single colour because they cannot be separately distinguished, for example due to small size or rapid motion.

Optimal colour stimuli: object colour stimuli corresponding to objects whose luminance factors have maximum possible values for each chromaticity when their spectral luminance factors do not exceed 1 for any wavelength. … For a given luminance factor, these colour stimuli define the maximum purity possible for non-fluorescent objects” (CIE, 2011, 17-851).

Paint-mixing path: path of a mixture of paints through colour space.

Palette: 1. surface used for paint-mixing. 2. the set of paints used by a painter.

Palette, double or split primary: Palette consisting of six paints corresponding to a “warm” and a “cool” version of each of the three historical primaries, red, yellow and blue.

Palette, limited: palette chosen to provide a restricted gamut of colours.

Penumbra: transitional zone at the edge of a cast shadow, typically broadest where it is furthest from the object casting the shadow..

Permanence: resistance to colour change on exposure to light and atmosphere.

Primary colour: 1. hues considered basic or simple in some system, such as the historical, additive, subtractive or psychological primaries. 2. the specific pigments of a painter’s palette (uncommon). 3. in additive colour technology and colourimetry, “primaries” include both specific physical lights and virtual lights derived mathematically from these (e.g. CIE X, Y and Z).

Primary colour, additive: optimal hues of lights for additive mixing processes: orange-red, yellowish green and blue or violet-blue, commonly known simply as red, green and blue.

Primary colour, historical: primary colours of traditional colour theory: red, yellow and blue.

Primary colour, psychological: alternative name for the four opponent hues: red, yellow, green and blue.

Primary colour, subtractive: optimal hues of colourants for subtractive mixing processes: cyan, magenta and yellow.

Reflectance: ratio of the reflected flux to the incident flux of radiation. Luminous relectance = reflectance of visible radiation i.e. light.

Reflection. diffuse: “diffusion by reflection in which, on the macroscopic scale, there is no regular reflection” (CIE, 2011, 17-305); also called body reflection. Most materials exhibit diffuse and specular reflection simultaneously.

Reflection, specular: “reflection in accordance with the laws of geometrical optics, without diffusion” (CIE, 2011, 17-1077, under “regular reflection”), for example producing the highlights a glossy surface, or the image in a mirror. Also called regular or interface reflection.

Related colour: colour perceived to belong to an area seen in relation to other colours (CIE 2011, 17-1080).

Saturation:  “colourfulness of an area judged in proportion to its brightness” (CIE 2011, 17-1136).  Saturation by this definition is the relative freedom from whitishness of the light from an area, and is our perception of the degree of broad-scale unevenness of the spectral power distribution of that light. Commonly confused with chroma.

Scattering: “process by which the spatial distribution of a beam of radiation is changed when it is deviated in many directions by a surface or by a medium, without change of frequency of its monochromatic components” (CIE, 2011, 17-1139). Also called diffusion.

Secondary colour: in traditional colour theory, a colour considered to contain only two of the three historical primaries (red, yellow and blue).

Shade: object colour perceived to consist of pure colour plus black, without any white content.

Shading series (or shadow series): Series of colours of uniform hue and saturation, differing only in lightness; appropriately arranged, such colours are seen as a uniformly coloured object under variable lighting.

Simultaneous contrast: perceived increase in colour difference when coloured areas are juxtaposed.

Spectral power distribution: spectral power per wavelength interval throughout the spectrum.

Spectral reflectance curve: graph showing the reflectance of an object at each wavelength of the spectrum.

Spectrum: originally introduced by Newton for the rainbow-like band of light produced by splitting a beam of sunlight using a prism, but see also edge spectra.

Subtractive mixture: process in which two or more colourants or filters each remove wavelengths from the light illuminating them; major factor in paint mixing.

Successive contrast: influence of a negative afterimage on the perceived colour of an area.

Terminator: line marking the limit of the direct fall of light on an object.

Tertiary colour: 1.in some versions of traditional colour theory, greyed colours, considered to “contain” all three historical primary colours (red, yellow and blue). 2. alternatively, the six third-order colours that lie between adjacent primaries and secondaries in a 12-hue traditional colour wheel (considered to “contain” just two primaries in unequal proportions).

Tint: object colour perceived to consist of a pure colour plus white, without any black content.

Tinting strength: power of a paint to influence the colour of mixtures.

Tone: 1. related in meaning to lightness/value, but usually held to increase with decreasing value. 2. object colour perceived to consist of a pure colour plus black and white, that is, all colours on a hue page except the tints and tones.

Tonal massing: traditional compositional strategy of grouping values into distinctive shapes in order to create a more striking tonal pattern.

Trichromatic model: model of human colour vision postulating three types of receptors in the retina.

Undertone: colour of a paint when spread thinly over a light ground or mixed with white.

Unrelated colour: colour perceived to belong to an area seen in isolation from other colours (CIE 2011, 17-1376).

Value: lightness of a light-reflecting object. Also called greyscale value.

Value, home: value at which a hue reaches its maximum chroma; also known as peak chroma value. Mainly a painters term used in relation to the gamut of paint mixtures.

Warm colours: colours deemed to have a psychological association with warmth (see colour temperature). In various accounts the warmest hue is usually at or near orange but ranges from yellow to red.

Whiteness: degree to which an object color is judged to approach a preferred or standard white.