One of the three primary colours of pigments in painting and traditional colour theory, as well as in the RGB colour model. On the spectrum of visible light, blue lies between violet and green. The eye perceives blue when observing light with a dominant wavelength between approximately 450 and 495 nanometres. Most blues contain a slight mixture of other colours: azure contains some green, while ultramarine contains some violet. Blue is commonly linked to the daytime sky or the sea. However, this is the result of an optical effect known as Rayleigh scattering.
Blue has been an important colour in art and decoration since ancient times. The semi-precious stone lapis lazuli was used in ancient Egypt for jewellery and ornament and later, in the Renaissance, to make the pigment ultramarine, the most expensive of all pigments. In the 8th century, Chinese artists used cobalt blue to colour fine porcelain. In the Middle Ages, European artists used it in the windows of cathedrals. Europeans wore clothing coloured with the vegetable dye woad until it was replaced by the finer indigo from America. In the 19th century, synthetic blue dyes and pigments gradually replaced organic dyes and mineral pigments. Dark blue became a common colour for military uniforms and later, in the late 20th century, for business suits.
As blue has been most commonly associated with harmony, it was chosen as the colour of the flags of the United Nations and the European Union. Surveys in the US and Europe show blue is also associated with faithfulness, confidence, intelligence, knowledge, calm and concentration, distance, infinity, the imagination, cold and, occasionally, sadness.
In American and European public opinion polls, it is the most popular colour, chosen by almost half of both men and women as their favourite colour. The same surveys also showed that blue is the colour most associated with the masculine, just ahead of black.