BLUE

Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 440–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal mixture of red and green light. On a colour wheel based on traditional colour theory (RYB), the complementary colour to blue is considered to be orange.
The English language commonly uses "blue" to refer to any colour from navy blue to cyan. The word itself is derived from the Old French word bleu.

Psychology

Blue often represents the human emotion of sadness, e.g. "He was feeling blue".
Music

The blues is a style of music originated by African Americans. Contrary to popular belief it is not called Blues because its lyrics are depressing but because its scale is inclusive of the "dark notes" or blue notes.

National colours

Blue is the national sports colour for India, as it denotes secularism.

Blue is the national colour used on flags of several countries surrounded by seas or oceans such as Australia, and also of Europe as a whole (not just the European Union that also uses the same flag).

Blue and white are the national colours of Scotland, Finland and the Central American countries of Guatemala, Honduras, El Salvador, and Nicaragua, as well as Greece, Israel and Nation of the Uniteds Organization using a light shade of blue symbolizing peace.

Blue and yellow are the national colours of Sweden, Kazakhstan, Ukraine, and Barbados, and along with green, of Brazil, and along with red, of Colombia, Venezuela, Ecuador, Chad, Romania, and Moldova.

Red and blue are the national colours of Romania, Liechtenstein and Haiti, and along with white (where it composed the French tricolour whose simple design or colors were taken by other countries), The United States, the United Kingdom, France, Luxembourg, Norway, the Czech Republic, Slovakia, the Netherlands, Costa Rica, Serbia, Croatia, Paraguay, Iceland, Panama, Russia, Cuba, Chile, Thailand, Dominican Republic, Puerto Rico, Liberia, Nepal, Slovenia, South Korea.

Mysticism

Psychics who claim to be able to observe the aura with their third eye report that someone with a blue aura is a person who is oriented toward spirituality. People with blue auras are said to be interested in social service work and to be in occupations such as social worker, counselor, teacher, writer, and psychologist.

And it ....is my favourite colour. It's the colour of the summer!