Red is a color at the lowest frequencies of light discernible by the human eye. Red light has a wavelength range of roughly 630-760 nm.

Red is an additive primary color, complementary to cyan. It was once considered to be a subtractive primary color, and is still sometimes described as such in non-scientific literature; however, the colors cyan, magenta and yellow are now known to be closer to the true subtractive primary colors detected by the eye, and are used in modern color printing.

Lower frequencies are called infrared, or far red.

A red filter used in black and white photography increases contrast in most scenes. For example, combined with a polarizer, it can turn the sky black. Films simulating the effects of infrared film (such as Ilford's SFX 200) do so by being much more sensitive to red than to other colors.

Oxygenated blood is red due to the presence of hemoglobin. Red light is the first to be absorbed by sea water, so that many fish and marine invertebrates that appear bright red are black in their native habitat.

Usage, symbolism, colloquial expressions

• Red is the color of warmth, for instance used to indicate warmer areas on a weather map, or for heat-related warnings.
• Red catches people's attention, and is often used to indicate danger or emergency.
  • Red is the color of heat and fire. Taps for hot water are often labeled red. Red is commonly the color of fire alarm boxes, fire extinguishers, and the firefighter profession itself.
  • Red denotes "stop" in, for instance, stop signs, traffic signals, brake lights, or the flashing lights of a school bus.
  • A Red Cross or Red Crescent signify medical personnel, facilities, or equipment, or the Geneva conventions.
  • Red indicates extreme danger on Western color-coded scales, such as wildfire hazard signs or the U.S. Homeland Security Advisory System.
  • In auto racing, a red flag signals all cars to immediately stop. The redline is the maximum speed an engine and its components can run.
  • Emergency exits on passenger aircraft are indicated by red signs and lighting.
  • "Redlining" is delineating a forbidden area (as on a map), for instance where a company denies or increases the cost of services, and is illegal in various circumstances in the U.S.
• In Association football (soccer), the referee shows a red card to a player who is being sent-off.
• Red is the color of both romantic and carnal love, thus the red of a valentine heart and of a "red-light district". But it may also denote anger, as in the expression seeing red, or embarrassment, as in being red-faced.
• Being the color of blood, red was associated with the god of war, Mars, and the reddish planet Mars became named after him. The phrase "red-blooded" describes someone who is audacious, robust, or virile; it is sometimes used to contrast with a cold or effete "blue blood" although the terms are unrelated in origin.
• Beginning with the Revolution of 1848, "Socialist" red was used as a color of European Revolutionaries, often in the form of the red flag. It was also used by Garibaldi's camicie rosse ("redshirts") in the Italian Risorgimento, and taken up by Leftist and generally radical groups, while the white of legitimist Bourbon partisans became associated with pre-World War I conservatives.
  • For instance the Civil War in Russia and the Civil War in Finland were fought between the "Red Army" and various "White Armies".
  • The identification of Communism with "Socialist" red (with the red flag being the primary color of the flag of the Soviet Union) and the red star being a Communist emblem led to such Cold War phrases as "the Red Menace" and "Red China" (distinguished from Nationalist China, "Free China," or Taiwan). The colour was also associated with political vehicles such as the Red Guard in China and the Red Guards during the Russian Revolution as well as with left wing paramilitary groups such as the Red Army Faction in Germany and the Japanese Red Army. .
  • Red remains associated with parties on the left of the political spectrum, with several notable exceptions (see "political_parties" below)

• In Chinese symbolism, red is the color of good luck and is used for decoration and wedding attire. Money in Chinese societies is traditionally given in red packets. See also The East is Red. Mao Zedong was sometimes referred to as a "red sun".
• Red ink is also used to denote debt - as well as losses on a balance sheet (hence the phrase, "in the red" usually indicates economic losses).
• In North American stock markets, red is used to denote a drop in stock prices. In East Asian stock markets, red is used to denote a rise in stock prices.
• In maps of political parties, red is traditionally used for the following parties:
  • Australia: Labor
  • Canada: Liberal Party of Canada
  • Germany: Sozialdemokratische Partei Deutschlands (Social Democratic party of Germany) and Partei des Demokratischen Sozialismus (Party of Democratic Socialism)
  • The Netherlands: Partij van de Arbeid (Socialist party)
  • United Kingdom: Labour Party
  • United States: Republican Party, hence states that vote Republican are referred to as red states as opposed to blue states which vote for the Democrats.
• Red is one of the Christmas colors, with green, white, or both.
• Red, along with yellow and orange, is thought to provoke hunger, hence its use in logos by food vendors.
• In the history of Japan red is the color of military flags used by the Heike (or Taira) clan and of the Genji (or Minamoto) clan, two clans that struggled for power at the close of the Heian era, in the late 12th century.
• Red is the last film in the Three Colors trilogy by Krzysztof Kieślowski.
• Red is the color claimed by the street gangs Bloods and Norteños.
• Red is the word for a fish of the genus Myripristis in Tobian.
• Red is an album by progressive rock band King Crimson.
• Red is the color of the lowest-value balls in snooker. There are 15 at the beginning of a game, each worth 1 point.
• In the original Star Trek series, red uniforms are for engineering and security personnel. (On away missions, ensigns in red uniform would often get killed). For Star Trek: The Next Generation onwards, red uniforms are for command personnel.

Variations

• Scarlet - a shade of red that tends towards red-orange and has no hint of blue
• Vermilion - a fiery shade of red that tends toward red-orange to a silghtly greater degree than scarlet, prepared from cinnabar, the artificial red sulphide of mercury used as a pigment
• Pink - a very light, unsaturated red, traditionally the color of pinks
• Maroon - a deep, dark, brownish (desaturated) red
• Venetian Red (also known as India Red or Indian Red) - A shade of brownish red prepared from sulphate of iron.
• Carmine - a dark, blue-tinged red traditionally the color of a dye made from the cochineal insect
• Rose is a range of colors on the blue side of red
  • Damask specifically refers to the color of the Damask rose.
• Crimson - a shade of red that has no hint of yellow and leans towards red-violet
• Cerise - another dark blue-red
• "Fire Engine Red" - an intense red commonly used on emergency vehicles
• "Jungle Red" was the nail polish color in The Women.
• Peach is a range of colors on the yellow side of red and generally tending toward a light tint.

See also

• List of colors