Short answers, pulled from the story.
Red corresponds to light with a wavelength of approximately 625 to 750 nanometers. It sits at the long-wavelength end of the visible spectrum, next to orange and opposite violet. Light beyond this range is called infrared and cannot be seen by the human eye, though it can be sensed as heat.
Paleoanthropologists found in 2000 that people at Pinnacle Point in South Africa were scraping and grinding ochre between 170,000 and 40,000 years ago, most likely to color their bodies. Red ochre was also used to paint a bison in the cave of Altamira in Spain, a work that dates to between 15,000 and 16,500 BC.
Red's association with labor movements grew through the 18th and 19th centuries, when striking English dock workers carried red flags and Parisian demonstrators used the color in the 1832 June Rebellion and the 1848 revolution. After the Paris Commune of 1871 adopted the red flag, Karl Marx and European socialist and communist movements embraced it. Soviet Russia raised the red flag after the Bolshevik Revolution in 1917, anchoring the color's political meaning globally.
Red was standardized as the international color for stop signs and traffic lights under the Vienna Convention on Road Signs and Signals of 1968. It was chosen primarily because of its universal association with danger and warning, and also because red is the brightest color in daytime conditions. The earlier German-type signage system established in 1927 had already used red for warning and prohibitory signs.
Allura Red AC is the most common synthetic red food coloring in use today, a red azo dye originally made from coal tar and now mostly derived from petroleum. It is banned in Denmark, Belgium, France, and Switzerland due to health concerns, particularly regarding children. In the United States, Red 3 dye was banned in 2025, and on the 30th of June 2010 the Center for Science in the Public Interest called on the FDA to ban Red 40.
Studies show that competitors wearing red are perceived as more dominant, with tae kwon do judges documented as favoring red-gear competitors over those in blue. Wearing red has been linked with increased performance outcomes in professional sport and multiplayer video games. However, controlled tests found no increase in actual testosterone or physical performance, suggesting the effect comes from perceived rather than real advantage.