Jeans are typically dyed blue with indigo, hence the name blue jeans. The indigo dye, which gives jeans their deep blue colour, has a long history.
Preparation of the dye tubs and the dye process itself are complicated and require a lot of work. The dye bath starts out a white-green colour, which only turns blue once the textile is exposed to oxygen. The more often the fabric is dyed, the deeper the blue becomes.
In 1826, French Jean Baptiste Guimet secretly developed a synthetic blue, which was put on the market at the end of the 19th century by the German company Badische Anilin und Soda Fabrik (BASF). Synthetic indigo soon exceeded the demand of traditionally produced dye.
The use of the word indigo could be confusing, as it refers to the dye itself, the colour of the dyed fabric, as well as the dye’s natural sources, the Woad (Isatis tinctoria L) and True Indigo (Indigofera tinctoria L). The two plants produce a similar blue dye – so similar that chemical analysis of historical textile cannot even tell whether it’s one or the other. However, pure indigo has a slightly red cast and can produce deep, brilliant blues that cannot be duplicated with any other dye.
Sources: Nouvelle de Nîmes Nº 5: The Denim Dictionary and the Tellason Denim Terminology