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 colours 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.

n the 17th century, Frederick William, Elector of Brandenburg, was one of the first rulers to give his army blue uniforms. The reasons were economic; the German states were trying to protect their pastel dye industry against competition from imported indigo dye. When Brandenburg became the Kingdom of Prussia in 1701, the uniform color was adopted by the Prussian army. Most German soldiers wore dark blue uniforms until the First World War, with the exception of the Bavarians, who wore light blue.