Yellow

• 1. Introduction

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  Yellow sits between green and orange on the spectrum of visible light, and it has shaped human civilization in ways that most people never consider. The oldest known painting of a yellow horse, found on the walls of the Lascaux cave in France, is estimated to be 17,000 years old. A single color has served as a mark of royalty, a badge of shame, a sign of cowardice, a symbol of wisdom, and the visual signature of an entire empire. How can one color carry such contradictory weight? The answer runs through chemistry, politics, astronomy, religion, and the biology of plants, and it begins deep underground with a lump of clay.

  Light with a dominant wavelength of roughly 575 to 585 nanometers produces the sensation of yellow in the human eye. In subtractive color systems, the kind used in painting and printing, yellow is a primary color. On a television or computer screen, it is a secondary color, created by combining red and green light at equal intensity. These two definitions already hint at a recurring theme: yellow is never quite what it seems. It means different things depending on the system you are using to look at it.

• 2. The Chemistry of Yellow

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  Carotenoids are the organic pigments responsible for the yellow of autumn leaves, egg yolks, bananas, daffodils, and canaries. They are found in the chloroplasts and chromoplasts of plants and in some bacteria, algae, and fungi. Their role is dual: they absorb light energy to support photosynthesis, and they shield green chlorophyll from damage caused by overexposure to light.

  In late summer, as daylight hours shorten and temperatures drop, the veins carrying fluids into and out of a leaf gradually close off. Water and mineral intake slows, then stops. Chlorophyll breaks down first. As the green fades, the yellow and red carotenoids that were always present in the leaf become visible, producing the autumn color that seems, paradoxically, to be a kind of dying brightness.

  Bananas follow a similar script. They are green when picked because of the chlorophyll in their skin. After picking, hormones in the fruit convert amino acids into ethylene gas. That gas stimulates a chain of enzymes that alter the color, texture, and flavor of the banana. The chlorophyll supply is cut off, the yellow carotenoids take over, and eventually the enzymes break down the cell walls entirely, turning the banana brown.

  Xanthophylls, the most common yellow pigments within the carotenoid family, take their name from the Greek words for yellow and leaf. They move through the food chain. The yellow of a chicken's egg yolk, fat, and skin comes not from the chicken itself but from the xanthophylls, typically lutein, that farmers add to the chickens' feed.

  Saffron, one of the rarest and most expensive dyes in the world, also belongs to this broader carotenoid family. It is made from the dried red stigma of the crocus sativus flower, and it takes 150 flowers to yield a single gram of usable material. A 7th-century BC Assyrian botanical reference compiled under Ashurbanipal already documented its use. After the Buddha's death, his followers decreed that monks should wear robes the color of saffron, a tradition that has continued for more than two millennia.

• 3. Pigments, Dyes, and Toxins

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  Yellow ochre, a hydrated ferric oxide found in clays around the world, is non-toxic and has served painters since prehistoric times. The Egyptians paired it with orpiment, a brilliant but deadly pigment made from arsenic trisulfide. A small paintbox containing orpiment was found in the tomb of King Tutankhamun. Egyptian conventions were strict: men were painted with brown faces, women with yellow ochre or gold faces, reflecting the belief that the gods themselves had skin and bones made of gold.

  Orpiment, also called King's Yellow, remained in use until the 19th century, when cadmium yellow, first synthesized in 1820, and chrome yellow, first made in 1809, offered alternatives. Chrome yellow was derived from the mineral crocoite and used by artists in the earlier part of the 19th century, but its toxicity because of lead content eventually caused it to fall out of favor as well.

  Georges Seurat discovered the instability of some new synthetic pigments the hard way. He used zinc yellow, also called zinc chromate, in the light green lawns of his famous Sunday Afternoon on the Isle de la Grand Jatte, painted between 1884 and 1886. He did not know the pigment was highly unstable. Those areas quickly turned brown after the painting was finished.

  Vincent van Gogh had a different relationship with yellow. Writing to his sister from the south of France in 1888, he described the sunlight there as "bright sulfur yellow, pale lemon gold," and said he could only call it yellow for lack of a better word. He painted his famous sunflowers inside a small house he rented at 2 Place Lamartine in Arles, a house he described as painted the color of "buttery yellow." Van Gogh was also among the first artists to use commercially manufactured paints, combining traditional yellow ochre with the newer chrome yellow and cadmium yellow.

  Naples Yellow, or lead antimonate yellow, is one of the oldest synthetic pigments, derived from the mineral bindheimite. It was used extensively up to the 20th century before being replaced. Indian Yellow, a transparent fluorescent pigment used in oil paintings and watercolors, had an unusual origin: it was claimed to have been produced from the urine of Indian cows fed only on mango leaves.

• 4. Light, Stars, and Lasers

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  F. G. W. Struve became the first astronomer to classify stars by color in 1827. One of his categories was flavae, or yellow, which roughly corresponds to what modern classification calls the F5 to K0 spectral range. The Strömgren photometric system for stellar classification includes a yellow filter centered at a wavelength of 550 nanometers with a bandwidth of 20 to 30 nanometers.

  Yellow supergiants are rare. Stars of spectral classes F and G in the supergiant range are physically unstable, representing a transitional phase between blue and red supergiants rather than a stable state. A particular subset, the Cepheid variables, pulsate with a period directly proportional to their absolute magnitude. Because their pulsation period reveals their true brightness, astronomers can calculate their distance with great precision. Cepheid variables were used to determine distances both within and beyond the Milky Way. The most famous current example is Polaris, the North Pole star.

  On Earth, yellow lasers are harder to make and more expensive than most other laser colors. Commercial devices use a technique called diode pumped solid state technology. An infrared laser diode at 808 nanometers pumps a crystal of neodymium-doped material, causing it to emit at two frequencies simultaneously. That deeper infrared light then passes through a second crystal whose nonlinear properties combine the two beams into a single output at 593.5 nanometers, which is on the edge of true yellow. A variant of this approach, made available in 2010, produces a wavelength of 589 nanometers, which is considered a true yellow. Yellow lasers at 589 and 594 nanometers have found growing use in the field of optogenetics.

• 5. The Color of Power and Exclusion

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  In ancient China, bright yellow was the color of the Middle Kingdom itself. The palace of the Emperor was considered the exact center of the world. After the Song dynasty, laws banned certain bright yellow shades for anyone other than the emperor. Puyi, the last emperor of China, born in 1906 and died in 1967, described in his memoirs how every object surrounding him as a child was yellow. He wrote that it made him understand from his earliest age that he was of a unique essence, instilling in him the consciousness of what he called his "celestial nature."

  Distinguished visitors to China were welcomed on a yellow carpet, not a red one. Yellow was so central to imperial identity that the legendary first emperor of China was called the Yellow Emperor.

  In Europe, yellow carried darker associations. During the post-classical period, yellow became firmly associated with Judas Iscariot, even though the Bible never describes his clothing. That link extended to a broader marking of outsiders. In 16th-century Spain, those accused of heresy who refused to renounce their views were compelled to appear before the Spanish Inquisition in a yellow cape. In the 20th century, Jews in Nazi Germany and Nazi-occupied countries were required to sew yellow triangles bearing the Star of David onto their clothing.

  The 1986 People Power Revolution in the Philippines became known as the Yellow Revolution because of the yellow ribbons that protesters carried during the demonstrations. Political parties including UNIDO, PDP-Laban, and the Liberal Party adopted yellow as their color. Decades later, the color became a pejorative used by supporters of Ferdinand Marcos and Rodrigo Duterte against those same opposition groups, showing how a color's political meaning can reverse inside a single generation.

  In France in November and December of 2018, an opposition movement called the Yellow Vests took to the streets to protest the fiscal policies of President Emmanuel Macron. They wore the yellow safety vests that French motorists are required by law to keep in their vehicles.

• 6. Visibility, Warning, and the Road

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  Yellow is the most visible color from a distance, a fact that has quietly organized the physical world most people move through every day. Taxi entrepreneur John D. Hertz painted his fleet yellow in Chicago after a University of Chicago study claimed that yellow was the color most easily seen at a distance. That decision started a tradition that spread across countries.

  School buses in Canada and the United States are painted a uniform shade known as school bus yellow. In the 1960s, Pickett Brand developed what it called Eye Saver Yellow, a specific color at Angstrom 5600 that reflects long-wavelength rays, for its slide rules. The Occupational Safety and Health Administration uses Pantone 116 as its standard color for general warning. The Federal Highway Administration uses yellow on highway signage to communicate caution. A yellow traffic light means slow down. A yellow card in a soccer match means a formal warning. A yellow card in rugby sends a player temporarily to the sin bin.

  The yellow jersey awarded to the leader in certain cycling stage races, called the maillot jaune, traces back to the Tour de France. The sponsoring newspaper, L'Auto, later known as L'Equipe, was printed on distinctive yellow newsprint, and the leader's jersey took its color from the paper.

  In maritime signaling, a yellow flag represents the letter Q in international signal flags. It also means that a ship is asserting it does not need to be quarantined.

• 7. Yellow in Language and Culture

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  The English word yellow comes from the Old English geolu or geolwe, derived from the Proto-Germanic word gelwaz. Its Indo-European root, gʰel-, carries a dual meaning: both bright and gleaming, and to cry out. That same root connects yellow to the words gold and yell. The oldest known use of the word in written English appears in the Epinal Glossary, dated to the year 700.

  In Italian, giallo refers to crime fiction and true crime stories. The association dates to around 1930, when the first series of Italian crime novels were published with yellow covers. The Italian term has since entered academic and critical vocabulary to describe the entire genre.

  In the United States, yellow journalism refers to news that presents limited research, a term that traces back to a cartoon character named Mickey Dugen, better known as the Yellow Kid, who appeared in New York newspaper comic strips beginning in 1895. The strip exploited a new color printing process using magenta, cyan, yellow, and black to create full color images. The Yellow Kid wore a yellow nightshirt, and the sensational style of the papers that ran his strip eventually lent his color to the whole genre.

  The expression yellow-belly, meaning coward, comes from the 19th century in the United States. Its precise origin is unclear, though it may connect to the color of sickness, suggesting a person who lacks strength.

  In China, the term yellow movie refers to pornographic films, functioning as the equivalent of the English blue movie. This usage became part of broader pop culture through a 2007 internet meme based on the phrase "very erotic very violent."

  In music, the Beatles released Yellow Submarine on their 1966 album Revolver, and United Artists followed in 1968 with an animated film of the same name. Coldplay reached worldwide audiences with their 2000 single Yellow. The Yellow River Piano Concerto, arranged by a collaboration of musicians including Yin Chengzong and Chu Wanghua, premiered in 1969 during the Cultural Revolution in China.