Blue is the color of the clear sky and the deep sea. On the optical spectrum, blue is located between violet and green. The modern English word blue comes from Middle English bleu or blewe, from the Old French bleu, a word of Germanic origin, related to the Old High German word blao.

Hues of blue include indigo and ultramarine, closer to violet; pure blue, without any mixture of other colors; Cyan, which is midway on the spectrum between blue and green, and the other blue-greens turquoise, teal, and aquamarine.

Blues also vary 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Blue pigments were originally made from minerals such as lapis lazuli, cobalt and azurite, and blue dyes were made from plants; usually woad in Europe, and Indigofera tinctoria, or True indigo,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.