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For much of the 19th century artistic colour theory either lagged behind scientific understanding or was augmented by science books written for the lay public, in particular ''Modern Chromatics'' (1879) by the American physicist [[Ogden Rood]], and early colour atlases developed by [[Albert Munsell]] (''Munsell Book of Color'', 1915, see [[Munsell colour system]]) and [[Wilhelm Ostwald]] (Color Atlas, 1919). Major advances were made in the early 20th century by artists teaching or associated with the German [[Bauhaus]], in particular [[Wassily Kandinsky]], [[Johannes Itten]], [[Faber Birren]] and [[Josef Albers]], whose writings mix speculation with an empirical or demonstration-based study of colour design principles.
== ColorColour mixing ==
{{Main|ColorColour mixing}}
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One of the earliest purposes of colorcolour theory was to establish rules governing the mixing of pigments.
Traditional colorcolour theory was built around "pure" or ideal colorscolours, characterizedcharacterised by different sensory experiences rather than attributes of the physical world. This has led to several inaccuracies in traditional colorcolour theory principles that are not always remedied in modern formulations.<ref>{{Cite web|title=handprint: colormakingcolourmaking attributes|url=https://www.handprint.com/HP/WCL/color6.html|access-date=2021-07-31|website=www.handprint.com}}</ref> Another issue has been the tendency to describe colorcolour effects holistically or categorically, for example as a contrast between "yellow" and "blue" conceived as generic colorscolours instead of the three colorcolour attributes generally considered by colorcolour science: [[hue]], [[colorfulnesscolourfulness]] and [[lightness]]. These confusions are partly historical and arose in scientific uncertainty about colorcolour perception that was not resolved until the late 19th century when artistic notions were already entrenched. They also arise from the attempt to describe the highly contextual and flexible behavior of colorcolour perception in terms of abstract colorcolour sensations that can be generated equivalently by any [[visual media]].{{Citation needed|date=September 2022}}
=== Primary colorscolours ===
{{Main|Primary colorcolour}}
[[File:Color star-en (tertiary names).svg|thumb|Primary, secondary, and tertiary colorscolours of the [[RYB colorcolour model]]]]
ColorColour theory asserts three pure primary colorscolours that can be used to mix all possible colorscolours. These are sometimes considered as red, yellow and blue ([[RYB colorcolour model|RYB]]) or as red, green and blue ([[RGB colorcolour model|RGB]]).{{Citation needed|date=April 2024}} Ostensibly, any failure of specific [[paint]]s or [[ink]]s to match this ideal performance is due to the impurity or imperfection of the colorantscolourants. In contrast, modern colorcolour science does not recognizerecognise universal primary colorscolours (no finite combination of colorscolours can produce all other colorscolours) and only uses primary colorscolours to define a given [[colorcolour space]].<ref name="Handprint1"/> Any three primary colorscolours can mix only a limited range of colorscolours, called a [[gamut]], which is always smaller (contains fewer colorscolours) than the full range of colorscolours humans can perceive.<ref>{{Cite web|title=Traditional and Modern Colour Theory Part 1: Modern Colour Theory|url=http://www.huevaluechroma.com/112.php|access-date=2021-10-15|language=en-AU}}</ref> Primary colorscolours also can't be made from other colorscolours as they are inherently pure and distinct.<ref>{{Cite web|title=3 Basic Primary ColorsColours {{!}} Additive and Subtractive ColorColour Mixing|url=https://www.geeksforgeeks.org/primary-colors/|date=February 28, 2024 |access-date=2025-01-10|language=en-USA}}</ref>
=== Complementary colorscolours ===
{{Main|Complementary colorscolours}}
[[File:Chevreul's RYB chromatic diagram.png|upright=1.35|thumb|[[Michel Eugène Chevreul|Chevreul]]'s 1855 "chromatic diagram" based on the [[RYB colorcolour model]], showing [[complementary colorscolours]] and other relationships]]
For the mixing of coloredcoloured light, [[Isaac Newton]]'s [[colorcolour wheel]] is often used to describe complementary colorscolours, which are colorscolours that cancel each other's hue to produce an achromatic (white, gray or black) light mixture. Newton offered as a conjecture that colorscolours exactly opposite one another on the hue circle cancel out each other's hue; this concept was demonstrated more thoroughly in the 19th century. An example of [[complementary colorscolours]] would be magenta and green.{{citation needed|date=April 2024}}
A key assumption in Newton's hue circle was that the "fiery" or maximum saturated hues are located on the outer circumference of the circle, while achromatic white is at the centercentre. Then the saturation of the mixture of two spectral hues was predicted by the straight line between them; the mixture of three colorscolours was predicted by the "centercentre of gravity" or centroid of three triangle points, and so on.
According to traditional colorcolour theory based on [[subtractive primary colorcolour]]s and the RYB colorcolour model, yellow mixed with purple, orange mixed with blue, or red mixed with green produces an equivalent graygrey and are the painter's complementary colorscolours.
One reason the artist's primary colorscolours work at all is due to the imperfect pigments being used have sloped absorption curves and change colorcolour with concentration. A pigment that is pure red at high concentrations can behave more like magenta at low concentrations. This allows it to make purples that would otherwise be impossible. Likewise, a blue that is ultramarine at high concentrations appears cyan at low concentrations, allowing it to be used to mix green. [[Chromium]] red pigments can appear orange, and then yellow, as the concentration is reduced. It is even possible to mix very low concentrations of the blue mentioned and the chromium red to get a greenish colorcolour. This works much better with oil colorscolours than it does with watercolorswatercolours and dyes.
The old primaries depend on sloped [[Absorption spectroscopy|absorption]] curves and pigment leakages to work, while newer scientifically derived ones depend solely on controlling the amount of absorption in certain parts of the [[spectrum]].
{{Main|Tint, shade and tone}}
When mixing pigments, a colorcolour is produced which is always darker and lower in chroma, or saturation, than the parent colorscolours. This moves the mixed colorcolour toward a neutral color—acolour—a graygrey or near-black. Lights are made brighter or dimmer by adjusting their brightness, or energy level; in painting, lightness is adjusted through mixture with white, black, or a colorcolour's complement.
It is common among some painters to darken a paint colorcolour by adding black paint—producing colorscolours called ''shades''—or lighten a colorcolour by adding white—producing colorscolours called ''tints''. However, it is not always the best way for representational painting, as an unfortunate result is for colorscolours to also shift in hue. For instance, darkening a colorcolour by adding black can cause colorscolours such as yellows, reds, and oranges, to shift toward the greenish or bluish part of the spectrum. Lightening a colorcolour by adding white can cause a shift towards blue when mixed with reds and oranges. Another practice when darkening a colorcolour is to use its opposite, or complementary, colorcolour (e.g. purplish-red added to yellowish-green) to neutralizeneutralise it without a shift in hue and darken it if the additive colorcolour is darker than the parent colorcolour. When lightening a colorcolour this hue shift can be corrected with the addition of a small amount of an adjacent colorcolour to bring the hue of the mixture back in line with the parent colorcolour (e.g. adding a small amount of orange to a mixture of red and white will correct the tendency of this mixture to shift slightly towards the blue end of the spectrum).
=== Split primary palette ===
The split-primary palette is a colorcolour-wheel model that relies on misconceptions to attempt to explain the unsatisfactory results produced when mixing the traditional primary colorscolours, red, yellow, and blue.
Painters have long considered red, yellow, and blue to be primary colorscolours. In practice, however, some of the mixtures produced from these colorscolours lack [[ColorfulnessColourfulness|chromatic intensity]]. Rather than adopt a [[CMY colorcolour model|more effective set of primary colorscolours]],<ref>{{cite web |last1=Baird |first1=Christopher S. |title=Associate Professor of Physics |url=https://www.wtamu.edu/~cbaird/sq/2015/01/22/why-are-red-yellow-and-blue-the-primary-colors-in-painting-but-computer-screens-use-red-green-and-blue/ |website=Science Questions with Surprising Answers |publisher=West Texas A&M University |access-date=12 June 2024}}</ref> proponents of split-primary theory explain this lack of chroma by the purported presence of impurities, small amounts of other colorscolours in the paints, or biases away from the ideal primary toward one or the other of the adjacent colorscolours. Every red paint, for example, is said to be tainted with, or biased toward, either blue or yellow, every blue paint toward either red or green, and every yellow toward either green or orange. These biases are said to result in mixtures that contain sets of [[complementary colorscolours]], darkening the resulting colorcolour. To obtain vivid mixed colorscolours, according to split-primary theory, it is necessary to employ two primary colorscolours whose biases both fall in the direction, on the colorcolour wheel, of the colorcolour to be mixed, combining, for example, green-biased blue and green-biased yellow to make bright green. Based on this reasoning, proponents of split-primary theory conclude that two versions of each primary colorcolour, often called "cool" and "warm," are needed in order to mix a wide [[gamut]] of high-chroma colorscolours.<ref>{{cite web |last1=Kemp |first1=Will |title=The Hidden Hues of Colour Mixing |url=https://willkempartschool.com/the-hidden-secret-of-colour-mixing/ |website=willkempartschool.com |date=27 August 2011 |publisher=Will Kemp Art School |access-date=15 October 2023}}</ref><ref>{{cite web |last1=Short |first1=Susie |title=Working with a Split Primary Color Palette |url=https://danielsmith.com/artists/insights/susie-short-working-with-a-split-primary-color-palette/ |website=danielsmith.com |date=21 July 2022 |publisher=Daniel Smith |access-date=15 October 2023}}</ref>
In fact, the perceived bias of colorscolours is not due to impurity. Rather, the appearance of any given colorantcolourant is inherent to its chemical and physical properties, and its purity unrelated to whether it conforms to our arbitrary conception of an ideal hue. Moreover, the identity of gamut-optimizingoptimising primary colorscolours is determined by the physiology of [[Trichromacy|human colorcolour vision]]. Although no set of three primary paints can be mixed to obtain the complete colorcolour gamut perceived by humans, red, yellow, and blue are a poor choice if high-chroma mixtures are desired. This is because painting is a [[subtractive colorcolour]] process, for which red and blue are secondary, not primary, colorscolours.
Although flawed in principle,<ref>{{cite web |last1=Lucariello |first1=Joan |last2=Naff |first2=David |title=How Do I Get My Students Over Their Alternative Conceptions (Misconceptions) for Learning? Applications of Psychological Science to Teaching and Learning modules |url=https://www.apa.org/education-career/k12/misconceptions |website=APA.org |publisher=American Psychological Association |access-date=12 August 2024}}</ref> the split-primary system can be successful in practice, because the recommended blue-biased red and green-biased blue positions are often filled by near approximations of magenta and cyan, respectively, while orange-biased red and violet-biased blue serve as secondary colorscolours, tending to further widen the mixable gamut.
This system is in effect a simplified version of Newton's geometrical rule that colorscolours closer together on the hue circle will produce more vibrant mixtures. A mixture produced from two primary colorscolours, however, will be much more highly saturated than one produced from two secondary colorscolours, even though the pairs are the same distance apart on the hue circle, revealing the limitations of the circular model in the prediction of colorcolour-mixing results. For example, a mixture of magenta and cyan inks or paints will produce vivid blues and violets, whereas a mixture of red and blue inks or paints will produce darkened violets and purples, even though the angular distance separating magenta and cyan is the same as that separating red and blue.
== Color contrast ==
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