Color

• 1. Introduction

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  Color is not a property that lives inside an object. It is the visual perception produced when light activates the cone cells in your eye. A red apple is not truly red. It scatters certain wavelengths back to you while absorbing the rest. The brain does the rest of the work. This single fact unsettles something most people take for granted. If color is a sensation rather than a substance, then where does it come from, and why does it look the way it does? Bees see ultraviolet that no human will ever witness. Half of all human females may carry four cone classes instead of three. A mantis shrimp carries twelve. The visible spectrum runs from roughly 390 nanometers to 700 nanometers, and that narrow band is all most of us will ever know. What follows traces how light becomes sensation, how that sensation is split and processed, and how culture decides where one color ends and the next begins.

• 2. Light And Matter

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  Electromagnetic radiation carries two defining traits: its wavelength and its intensity. When that wavelength falls within the range humans can perceive, it becomes what we call visible light. Most light sources emit many wavelengths at once. A source's spectrum is the distribution that records its intensity at each one. There are far more possible spectral combinations than there are color sensations a person can have. Because of this, a color can be defined formally as a class of spectra that all produce the same sensation. Within each such class, the members are called metamers. Two utterly different mixtures of wavelengths can look identical to the eye. The physical color of an object depends on how it absorbs and scatters the light striking it. A transparent object lets nearly all light pass through, so it appears colorless. An opaque object absorbs or reflects instead. A translucent object scatters or absorbs certain wavelengths through internal scattering, and the absorbed light is often dissipated as heat. Isaac Newton named the familiar colors of the rainbow using the Latin word for appearance, in 1671. Those pure spectral colors, produced by light of a single wavelength, sit at 100 percent purity and are fully saturated, the most vivid that light alone can offer.

• 3. Three Receptors

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  Thomas Young, in 1801, proposed that the eye could not possibly hold hundreds of receptors each tuned to one wavelength. He suggested instead that color experience arises from the mixing of just three receptor outputs. Young's trichromatic theory was later confirmed by James Clerk Maxwell, who demonstrated experimentally that any color could be matched with a combination of three lights. Hermann von Helmholtz refined the work and credited Maxwell for reviving Young's neglected insight. Helmholtz wrote that Young's theory of color sensations remained unnoticed until Maxwell directed attention to it, after confirming Newton's law of mixture experimentally in 1856. The human retina holds three types of cone. S cones respond most to blue or blue-violet light near 450 nanometers. M cones answer to green near 540 nanometers. L cones peak around 570 nanometers, in greenish yellow. Each cone obeys the principle of univariance: its output depends only on the total light falling on it across all wavelengths. The three signals together are called tristimulus values. The curves overlap so heavily that it is impossible to stimulate the green cones alone. Out of these constraints, it has been estimated that humans can distinguish roughly 10 million different colors.

• 4. Opposing Channels

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  Ewald Hering, working at the same time as Helmholtz, noticed that color blindness and afterimages arrive in opposing pairs: red-green, blue-orange, yellow-violet, and black-white. His opponent process theory seemed to contradict the trichromatic view. In 1957, Hurvich and Jameson reconciled the two. They showed that retinal processing matches the trichromatic theory, while processing at the lateral geniculate nucleus follows the opponent theory. Color leaves the eye through three opponent channels: red-green, blue-yellow, and a black-white luminance channel. This structure explains why no one ever perceives a reddish green or a yellowish blue. From the V1 blobs, color information travels to the second visual area, V2, where the most strongly color-tuned cells cluster in thin stripes that stain for the enzyme cytochrome oxidase. Those neurons then synapse onto cells in the extended V4. Semir Zeki first proposed that V4 was dedicated exclusively to color, then showed it could be subdivided into subregions packed with color cells. Color processing in extended V4 happens in millimeter-sized modules called globs. That is where color is first assembled into the full range of hues found in color space.

• 5. Beyond Trichromacy

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  Congenital red-green color blindness, the most common form, affects roughly 8 percent of males. In its strongest form, dichromacy, blue and purple, green and yellow, teal, and gray collapse into colors of confusion. The deficiency can be mild anomalous trichromacy, a moderate loss of an entire channel, or complete monochromacy with no color at all. Cerebral achromatopsia arises instead from neural anomalies in the brain's visual regions. Most mammals are dichromatic, carrying only two cones, which makes human trichromacy unusual among them. Most other vertebrates run richer. Birds, reptiles, amphibians, and bony fish are typically tetrachromatic with four cones, letting them see as distinct two colors a human reads as metamers. The mantis shrimp pushes further with twelve cones. Human tetrachromacy remains contentious. As many as half of all human females may express four cone classes in the retina, yet expressing them is not the same as using them. A distinction separates weak retinal tetrachromats from strong functional ones who can actually make the finer discriminations. Only one peer-reviewed report of a functional tetrachromat exists. Where the average person sees one million colors, such a person could see a hundred million.

• 6. Color And Mind

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  Pythagoras experienced synesthesia and left one of the first written accounts of the condition, around 550 BCE. He built mathematical equations for the musical notes that form a scale, such as the octave. In grapheme-color synesthesia, letters and numbers evoke color, while in chromesthesia, sounds do. Neuroimaging shows these experiences activate the same brain regions involved in ordinary color perception, proving they are real percepts reached by a non-standard route. About 4 percent of the population carries gene variants associated with the condition, and it can also follow brain damage, drugs, or sensory deprivation. Afterimages reveal another quirk of the system. After strong light desensitizes photoreceptors of one type, colors seen for the next few seconds lack that component, appearing in a complementary hue. Vincent van Gogh and other artists put this effect to deliberate use. The visual system also fights to keep color steady. Edwin H. Land studied this in the 1970s and proposed his retinex theory of color constancy, which explains how colors in a scene stay relatively constant as the illumination shifts. With a limited palette of red, yellow, black, and white, the eye fills the gaps: yellow and black read as green, red and black as purple, and pure gray takes on a bluish cast.

• 7. Reproducing Color

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  Most colors are not spectral colors at all. They are mixtures of wavelengths, often described by a dominant wavelength that names the single wavelength producing the closest sensation. Purples cannot be spectral by definition, since they mix red and violet from opposite ends of the spectrum. Black, gray, white, pink, tan, and magenta are all necessarily non-spectral. Because the eye reduces any light to three components, most perceptions can be rebuilt from three primaries, the basis of color reproduction in photography, printing, and television. No mixture matches a true spectral color exactly. Mixing green light at 530 nanometers with blue at 460 nanometers yields a cyan that is slightly desaturated, because the red receptor answers the mixture more than it would a pure cyan at 485 nanometers. The range a system can reproduce is its gamut, mapped on the CIE chromaticity diagram. In 1931, the International Commission on Illumination built the mathematical model that specifies every color with three numbers. Additive color mixes light, using red, green, and blue in projectors and televisions. Subtractive color uses dyes, inks, and pigments to absorb wavelengths, which is why red paint under blue light can appear black. Structural color works differently again, arising from interference rather than pigment, giving the blue jay its feathers and the peacock its iridescence. Isaac Newton and Robert Hooke both studied butterfly wings and beetle shells, and since 1942 electron micrography has driven products that exploit the effect.

• 8. Naming The Spectrum

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  The ROYGBIV mnemonic feels permanent, yet the spectral colors form a continuous band, and how language carves it up is a matter of culture and historical contingency. Even in English, the inclusion of indigo and cyan stays contentious, and where one term ends and the next begins is rarely agreed. Brent Berlin and Paul Kay traced this pattern in their 1969 study, Basic Color Terms: Their Universality and Evolution. Every language with just two basic color names splits dark and cool from bright and warm. The next term to emerge is usually red, then yellow or green. Languages with six basic colors carry black, white, red, green, blue, and yellow. Berlin and Kay followed the sequence up to twelve terms, adding gray, pink, orange, purple, brown, and azure, the last distinct from blue in Russian and Italian but not in English. Color also leaves measurable marks on the mind. Researchers at the University of Linz in Austria found that the color red significantly decreases cognitive functioning in men. Red and yellow together can induce hunger, an effect a number of chain restaurants have used. A photograph in black and white is slightly less memorable than one in color, and wearing bright colors makes a person more memorable to those they meet.