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.
In Japanese, the word for blue (青 ao) is often used for colours that English speakers would refer to as green, such as the colour of a traffic signal meaning "go".
(For more on this subject, see Distinguishing blue from green in language) Linguistic research indicates that languages do not begin by having a word for the colour blue.
This method could produce almost all the colours in the spectrum with reasonable accuracy.
In the 19th century the Scottish physicist James Clerk Maxwell found a new way of explaining colours, by the wavelength of their light.
Today most blue pigments and dyes are made by a chemical process. Several languages, including Japanese, Thai, Korean, and Lakota Sioux, use the same word to describe blue and green.
For example, in Vietnamese the colour of both tree leaves and the sky is xanh.
Distant objects appear more blue because of another optical effect called atmospheric perspective.
Blue has been an important colour in art and decoration since ancient times.
Colour names often developed individually in natural languages, typically beginning with black and white (or dark and light), and then adding red, and only much later – usually as the last main category of color accepted in a language – adding the colour blue, probably when blue pigments could be manufactured reliably in the culture using that language.
Human eyes perceive blue when observing light which has a dominant wavelength of roughly 450–495 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.