Red color has a predominant light wavelength of roughly 620–740 nanometers.

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Reds can vary in shade from very light pink to very dark maroon or burgundy; and in hue from the bright orange-red scarlet or vermilion to the bluish-red crimson. In nature, the red color of blood comes from hemoglobin, the iron-containing protein found in the red blood cells of all vertebrates.

The red color of the Grand Canyon and other geological features is caused by hematite or red ochre, both forms of iron oxide. The red sky at sunset and sunrise is caused by an optical effect known as Rayleigh scattering, which, when the sun is low or below the horizon, increases the red-wavelength light that reaches the eye.

The color of autumn leaves is caused by pigments called anthocyanins, which are produced towards the end of summer, when the green chlorophyll is no longer produced.

One to two percent of the human population has red hair; the color is produced by high levels of the reddish pigment pheomelanin (which also accounts for the red color of the lips) and relatively low levels of the dark pigment eumelanin.

Since red is the color of blood, it has historically been associated with sacrifice, danger and courage.

Modern surveys in the United States and Europe show red is also the color most commonly associated with heat, activity, passion, sexuality, anger, love and joy.In China, India and many other Asian countries it is the color of symbolizing happiness and good fortune.The word can be further traced to the Proto-Germanic rauthaz and the Proto-Indo European root rewdʰ-. In the Akkadian language of Ancient Mesopotamia and in the modern Inuit language of Inuit, the word for red is the same word as "like blood".In Portuguese the word for red is vermelho, which comes from Latin "vermiculus", meaning "little worm".Thus Red Square in Moscow, named long before the Russian Revolution, meant simply "Beautiful Square".(Lists of shades of red and shades of pink are found at the end of this article.) Inside cave 13B at Pinnacle Point, an archeological site found on the coast of South Africa, paleoanthropologists in 2000 found evidence that, between 170,000 and 40,000 years ago, Late Stone Age people were scraping and grinding ochre, a clay colored red by iron oxide, probably with the intention of using it to color their bodies.