Blue is one of the three primary colours of pigments in painting and traditional colour theory, as well as in the RGB colour model. It lies between violet and green on the spectrum of visible light. 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 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 eighth century Chinese artists used cobalt blue to colour fine blue and white 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 mineral pigments and synthetic dyes. Dark blue became a common colour for military uniforms and later, in the late 20th century, for business suits. Because blue has commonly been 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 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 the most popular colour, chosen by almost half of both men and women as their favourite colour. The same surveys also showed that blue was the colour most associated with the masculine, just ahead of black, and was also the colour most associated with intelligence, knowledge, calm and concentration.
Blue also varies 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Blue pigments were originally made from minerals such as lapis lazuli, cobalt and azurite, and blue dyes were made from plants; usually woad in Europe, and Indigofera tinctoria, or true indigo,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.
Blue pigments were made from minerals, especially lapis lazuli and azurite. These minerals were crushed, ground into powder, and then mixed with a quick-drying binding agent, such as egg yolk (tempera painting); or with a slow-drying oil, such as linseed oil, for oil painting. To make blue stained glass, cobalt blue pigment was mixed with the glass. Other common blue pigments made from minerals are ultramarine, cerulean blue, and Prussian blue.
The invention of new synthetic pigments in the 18th and 19th centuries considerably brightened and expanded the palette of painters. J.M.W. Turner experimented with the new cobalt blue, and of the twenty colours most used by the Impressionists, twelve were new and synthetic colours, including cobalt blue, ultramarine and cerulean blue.
Monet and the impressionists were among the first to observe that shadows were full of colour. In his La Gare Saint-Lazare, the grey smoke, vapour and dark shadows are actually composed of mixtures of bright pigment, including cobalt blue, cerulean blue, synthetic ultramarine, emerald green, Guillet green, chrome yellow, vermilion and ecarlate red.
“The dark blue sky is spotted with clouds of an even darker blue than the fundamental blue of intense cobalt, and others of a lighter blue, like the bluish white of the Milky Way … the sea was very dark ultramarine, the shore a sort of violet and of light red as I see it, and on the dunes, a few bushes of Prussian blue.” Van Gogh