Short answers, pulled from the story.
Human eyes detect light with wavelengths between 625 and 740 nanometers as the color red. This specific range sits at the long end of the visible spectrum next to orange and opposite violet.
Paleoanthropologists discovered evidence inside cave 13B at Pinnacle Point on the South African coast showing people scraping ochre between 170,000 and 40,000 years ago. Late Stone Age groups likely used this iron oxide clay to color their bodies for ceremonial purposes.
Allura Red AC became the most common synthetic food coloring today originally manufactured from coal tar but now mostly made from petroleum. European countries including Denmark Belgium France and Switzerland banned Allura Red AC for children until Sweden joined the European Union in 1994.
Exposure to red decreases performance on cognitive tasks like IQ tests and elicits aversion in psychological achievement contexts. Teachers at a primary school in the UK were instructed not to mark children's work in red ink because it encourages a negative approach.
Soviet Russia adopted a red flag following the Bolshevik Revolution in 1917 which remained throughout the entire history of the Soviet Union until its fall in 1991. The People's Republic of China adopted its own red flag after the Chinese Communist Revolution when the Party took power in 1949.
The English Parliament's New Model Army adopted red military uniforms in 1645 which ordinary soldiers wore dyed with madder while officers wore scarlet coats dyed with cochineal. British soldiers became known as red coats because of these distinctive garments worn until the outbreak of World War I in August 1914.