Blue is the colour of the clear sky and the deep sea. On the optical spectrum, blue is located between violet and green.

Blue was a latecomer among colors used in art and decoration. Reds, blacks, browns, and ochres are found in cave paintings from the Upper Paleolithic period, but not blue. Blue was also not used for dyeing fabric until long after red, ochre, pink and purple. This is probably due to the perennial difficulty of making good blue dyes and pigments. The earliest known blue dyes were made from plants - woad in Europe, indigo in Asia and Africa, while blue pigments were made from minerals, usually either lapis lazuli or azurite.

In Egypt, blue was associated with the sky and with divinity. The Egyptian god Amun could make his skin blue so that he could fly, invisible, across the sky. Blue could also protect against evil; many people around the Mediterranean still wear a blue amulet, representing the eye of God, to protect them from misfortune.