Blue is the colour 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 appear 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 nanometers.

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, or range, of colours. Red and blue mixed together form violet, blue and yellow together form green.

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. 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. There is a prevalent misconception that Rayleigh scattering also explains blue eyes – and that there is no blue pigment in blue eyes – but this is not true. The irises of blue eyed people do contain pigment (actually dark brown melanin) and the effect is caused by scattering of blue wavelengths known as the Tyndall effect.

Our understanding, and search for, what might be perceived as the ‘bluest’ blue is complicated by terminology; for example by the use of the term indigo, a colour that is traditionally regarded as separate within the visible spectrum, as well as one of the seven colours of the rainbow: the colour between blue and violet. Although traditionally considered one of seven major spectral colours, sources differ as to the actual position of indigo within the electromagnetic spectrum, but it is generally defined as a deep and bright colour close to the colour wheel blue (a primary colour in the RGB colour model*), as well as to some variants of ultramarine. Modern sources place indigo in the spectrum between 420 and 450 nanometers, which lies on the short-wave side of colour wheel (RGB) blue, towards (spectral) violet.

However, the correspondence of this definition with the colours of actual indigo dyes is disputed. Optical scientists Hardy and Perrin list indigo as between 446 and 464 nm wavelength, which occupies a spectrum segment from roughly the colour wheel (RGB) blue extending to the long-wave side towards azure.

In western colour science, the search for the purest blue began during the 17th and 18th centuries, when chemists in Europe tried to discover a way to create synthetic blue pigments, avoiding the expense of importing and grinding lapis lazuli, azurite and other minerals. The Egyptians had created a synthetic colour, Egyptian blue, three thousand years BC, but the formula had been lost. The Chinese had also created synthetic blue pigments, but these formulae were not known in the west.

In 1709 a German druggist and pigment maker named Diesbach accidentally discovered a new blue while experimenting with potassium and iron sulphides. The new colour was first called Berlin blue, but later became known as Prussian blue. By 1710 it was being used by the French painter Antoine Watteau, and later his successor Nicolas Lancret. It became immensely popular for the manufacture of wallpaper, and in the 19th century was widely used by French impressionist painters. Beginning in the 1820s, Prussian blue was imported into Japan through the port of Nagasaki. It was called bero-ai, or Berlin blue, and it became popular because it did not fade like traditional Japanese blue pigment, ai-gami, made from the dayflower. Prussian blue was used by both Hokusai, in his famous wave paintings, and also by Hiroshige.

In 1824 the Societé pour l’Encouragement d’Industrie in France offered a prize for the invention of an artificial ultramarine which could rival the natural colour made from lapis lazuli. The prize was won in 1826 by a chemist named Jean Baptiste Guimet, but he refused to reveal the formula of his colour. In 1828, another scientist, Christian Gmelin, then a professor of chemistry in Tübingen, found the process and made his formula public. This was the beginning of a new industry to manufacture artificial ultramarine, which eventually almost completely replaced the natural product.

In 1878 a German chemist named Von Baeyer discovered a synthetic substitute for indigotine, the active ingredient of indigo – a natural dye derived from the plant Indigofera tinctoria and related species. This product gradually replaced natural indigo, and after the end of the First World War, it brought an end to the trade of indigo from the East and West Indies. In 1901 a new synthetic blue dye, called Indanthrone Blue, was invented, which had even greater resistance to fading during washing or in the sun. This dye gradually replaced artificial indigo, whose production ceased in about 1970. Today almost all blue clothing is dyed with a form of Indanthrone Blue.

International Klein Blue (IKB) was developed by the artist Yves Klein (28 April 1928 – 6 June 1962) in collaboration with Edouard Adam, a Parisian art paint supplier whose shop is still in business on the Boulevard Edgar-Quinet in Montparnasse. The uniqueness of IKB does not derive from the ultramarine pigment, but rather from the matte, synthetic resin binder in which the colour is suspended, and which allows the pigment to maintain as much of its original qualities and intensity of colour as possible. The synthetic resin used in the binder is a polyvinyl acetate developed and marketed at the time under the name Rhodopas M or M60A by the French pharmaceutical company Rhône-Poulenc. Adam still sells the binder under the name “Médium Adam 25.”

In May 1960, Klein deposited a Soleau envelope, registering the paint formula under the name International Klein Blue (IKB) at the Institut National de la Propriété Industrielle (INPI), but he never patented IKB. Only valid under French law, a soleau enveloppe registers the date of invention, according to the depositor, prior to any legal patent application. The copy held by the INPI was destroyed in 1965. Klein’s own copy, which the INPI returned to him duly stamped is still extant.

Possibly the most recent discovery in the search for the purest blue is YInMn Blue (named after the elements that make it up: yttrium, indium, manganese), an inorganic blue pigment that was accidentally discovered by Professor Mas Subramanian and his team at Oregon State University in 2009. Andrew E. Smith, a graduate student in the team, was researching electrical properties of manganese oxides that were mixed with other precursors and heated to 1,093 °C (1,967 °F). YInMn Blue is noteworthy for its vibrant, near-perfect blue colour and unusually high NIRS (near-infrared spectroscopy) reflectance.

The new pigment will be commercialised by the Shepherd Color Company in 2016.

*An RGB color space is any additive colour space based on the RGB colour model, an additive colour model in which red, green, and blue light are added together in various ways to reproduce a broad array of colours. The name of the model comes from the initials of the three additive primary colours, red, green, and blue.