Blue is the colou180px-Starry_Night_Over_the_Rhoner between violet and green on the optical spectrum of visible light. Human eyes perceive blue when observing light with a wavelength between 450 and 495 nanometres. Blues with a higher frequency and thus a shorter wavelength gradually look more violet, while those with a lower frequency and a longer wavelength gradually appear more green. Pure blue, in the middle, has a wavelength of 470 nanometres. In painting and traditional colour theory, blue is one of the three primary colours of pigments, along with red and yellow, which can be mixed to form a wide gamut of colours. Red and blue mixed together form violet, blue and yellow together form green. Blue is also a primary colour in the RGB color model, used to create all the colors on the screen of a television or computer monitor.

The modern English word blue comes from Middle English bleu or blewe, from the Old French bleu, a word of Germanic origin, related to the Old High German word blao.[2] The clear sky and the deep sea appear blue because of an optical effect known as Rayleigh scattering. When sunlight passes through the atmosphere, the blue wavelengths are scattered more widely by the oxygen and nitrogen molecules, and more blue comes to our eyes. Rayleigh scattering also explains blue eyes; there is no blue pigment in blue eyes. Distant objects appear more blue because of another optical effect called atmospheric perspective.

Blue has been used for art, decoration and as a clothing dye since ancient times. The semi-precious stone lapis lazuli, coming from mines in Afghanistan, was used in ancient Egypt for jewelry and ornament and later, in The Renaissance, to make the pigment ultramarine, the most expensive of all pigments. In the Middle Ages, cobalt blue was used to colour the stained glass windows of cathedrals. Beginning in the 9th century, Chinese artist used cobalt to make fine blue and white porcelain. Blue dyes for clothing were made from woad in Europe and indigo in Asia and Africa. In 1828 a synthetic ultramarine pigment was developed, and synthetic blue dyes and pigments gradually replaced mineral pigments and vegetable dyes. Pierre-Auguste Renoir, Vincent van Gogh and other late 19th century painters used ultramarine and cobalt blue not just to depict nature, but to create moods and emotions. In the late 18th century and 19th century, blue became a popular colour for military uniforms and police uniforms. In the 20th century, because blue was commonly associated with harmony, it was chosen as the colour of the flags of the United Nations and the European Union. Toward the end of the 20th century, dark blue replaced black and grey as the most common colour for business suits, because it was seen as serious without being grim.

Surveys in the US and Europe show that blue is the colour most commonly associated with harmony, faithfulness, confidence, distance, infinity, the imagination, cold, and sometimes with sadness. In US and European public opinion polls it is overwhelmingly the most popular colour, chosen by almost half of both men and women as their favourite colour.

Blue is the colour of light between violet and green on the visible spectrum. Hues of blue include indigo and ultramarine, closer to violet; pure blue, without any mixture of other colours; Cyan, which is midway on the spectrum between blue and green, and the other blue-greens turquoise, teal, and aquamarine.

Blues also vary in shade or tint; darker shades of blue contain black or grey, while lighter tints contain white. Darker shades of blue include ultramarine, cobalt blue, navy blue, and Prussian blue; while lighter tints include sky blue, azure, and Egyptian blue. Blue pigments were originally made from minerals such as lapis lazuli, cobalt and azurite, and blue dyes were made from plants; usually woad in Europe, and Indigofera tinctoria, or True indigo, in Asia and Africa. Today most blue pigments and dyes are made by a chemical process.

In the ancient world

Blue was a latecomer among colours used in art and decoration, as well as language and literature.[5] Reds, blacks, browns, and ochres are found in cave paintings from the Upper Paleolithic period, but not blue. Blue was also not used for dyeing fabric until long after red, ochre, pink and purple. This is probably due to the perennial difficulty of making good blue dyes and pigments.[6] The earliest known blue dyes were made from plants – woad in Europe, indigo in Asia and Africa, while blue pigments were made from minerals, usually either lapis lazuli or azurite.

Lapis lazuli, a semi-precious stone, has been mined in Afghanistan for more than three thousand years, and was exported to all parts of the ancient world.[7] In Iran and Mesopotamia, it was used to make jewellery and vessels. In Egypt, it was used for the eyebrows on the funeral mask of King Tutankhamun (1341–1323 BC).

The cost of importing lapis lazuli by caravan across the desert from Afghanistan to Egypt was extremely high. Beginning in about 2500 BC, the ancient Egyptians began to produce their own blue pigment known as Egyptian blue, made by grinding silica, lime, copper and alkalai, and heating it to 800 or 900 °C (1,470 or 1,650 °F). This is considered the first synthetic pigment.  Egyptian blue was used to paint wood, papyrus and canvas, and was used to colour a glaze to make faience beads, inlays, and pots. It was particularly used in funeral statuary and figurines and in tomb paintings. Blue was considered a beneficial colour which would protect the dead against evil in the afterlife. Blue dye was also used to colour the cloth in which mummies were wrapped.imgres

In Egypt, blue was associated with the sky and with divinity. The Egyptian god Amun could make his skin blue so that he could fly, invisible, across the sky. Blue could also protect against evil; many people around the Mediterranean still wear a blue amulet, representing the eye of God, to protect them from misfortune.

Blue glass was manufactured in Mesopotamia and Egypt as early as 2500 BC, using the same copper ingredients as Egyptian blue pigment. They also added cobalt, which produced a deeper blue, the same blue produced in the Middle Ages in the stained glass windows of the cathedrals of Saint-Denis and Chartres.

The Ishtar Gate of ancient Babylon (604–562 BC) was decorated with deep blue glazed bricks used as a background for pictures of lions, dragons and aurochs.

The ancient Greeks classified colours by whether they were light or dark, rather than by their hue. The Greek word for dark blue, kyaneos, could also mean dark green, violet, black or brown. The ancient Greek word for a light blue, glaukos, also could mean light green, grey, or yellow.

The Greeks imported indigo dye from India, calling it indikon. They used Egyptian blue in the wall paintings of Knossos, in Crete, (2100 BC). It was not one of the four primary colours for Greek painting described by Pliny the Elder (red, yellow, black and white), but nonetheless it was used as a background colour behind the friezes on Greek temples and to colour the beards of Greek statues.

The Romans also imported indigo dye, but blue was the colour of working class clothing; the nobles and rich wore white, black, red or violet. Blue was considered the colour of mourning. It was also considered the colour of barbarians; Julius Caesar reported that the Celts and Germans dyed their faces blue to frighten their enemies, and tinted their hair blue when they grew old.

Nonetheless, the Romans made extensive use of blue for decoration. According to Vitruvius, they made dark blue pigment from indigo, and imported Egyptian blue pigment. The walls of Roman villas in Pompeii had frescoes of brilliant blue skies, and blue pigments were found in the shops of colour merchants. The Romans had many different words for varieties of blue, including caeruleus, caesius, glaucus, cyaneus, lividus, venetus, aerius, and ferreus, but two words, both of foreign origin, became the most enduring; blavus, from the Germanic word blau, which eventually became bleu or blue; and azureus, from the Arabic word lazaward, which became azure.

At the beginning of the 20th century, many artists recognised the emotional power of blue, and made it the central element of paintings. During his Blue Period (1901–1904) Pablo Picasso used blue and green, with hardly any warm colours, to create a melancholy mood. In Russia, the symbolist painter Pavel Kuznetsov and the Blue Rose art group (1906–1908) used blue to create a fantastic and exotic atmosphere. In Germany, Wassily Kandinsky and other Russian émigrés formed the art group called Der Blaue Reiter (The Blue Rider), and used blue to symbolise spirituality and eternity. Henri Matisse used intense blues to express the emotions he wanted viewers to feel. Matisse wrote, “A certain blue penetrates your soul.”

In the art of the second half of the 20th century, painters of the abstract expressionist movement began to use blue and other colours in pure form, without any attempt to represent anything, to inspire ideas and emotions. Painter Mark Rothko observed that colour was “only an instrument;” his interest was “in expressing human emotions tragedy, ecstasy, doom, and so on.”

In fashion, blue, particularly dark blue, was seen as a colour which was serious but not grim. In the mid-20th century, blue passed black as the most common colour of men’s business suits, the costume usually worn by political and business leaders. Public opinion polls in the United States and Europe showed that blue was the favourite colour of over fifty per cent of respondents. Green was far behind with twenty per cent, while white and red received about eight per cent each.

In 1873 a German immigrant in San Francisco, Levi Strauss, invented a sturdy kind of work trousers, made of denim fabric and coloured with indigo dye, called blue jeans. In 1935, they were raised to the level of high fashion by Vogue magazine. Beginning in the 1950s, they became an essential part of uniform of young people in the United States, Europe, and around the world.

Blue was also seen as a colour which was authoritative without being threatening. Following the Second World War, blue was adopted as the colour of important international organisations, including the United Nations, the Council of Europe, UNESCO, the European Union, and NATO. United Nations peacekeepers wear blue helmets to stress their peacekeeping role. Blue is used by the NATO Military Symbols for Land Based Systems to denote friendly forces, hence the term “blue on blue” for friendly fire, and Blue Force Tracking for location of friendly units. The People’s Liberation Army of China (formerly known as the “Red Army”) uses the term “Blue Army” to refer to hostile forces during exercises.

The 20th century saw the invention of new ways of creating blue, such as chemiluminescence, making blue light through a chemical reaction.

In the 20th century, it also became possible to own your own colour of blue. The French artist Yves Klein, with the help of a French paint dealer, created a specific blue called International Klein blue, which he patented. It was made of ultramarine combined with a resin called Rhodopa, which gave it a particularly brilliant colour. The baseball team the Los Angeles Dodgers developed its own blue, called Dodger blue, and several American universities invented new blues for their colours.