Indigo is a term that is used to describe the dye, which gives denim its rich, deep blue colour, the colour of the dyed denim as well as the name of the green plant from which the dye originally came from, True Indigo (Indigofera Tinctoria). This plant and the Woad both produce a blue dye which has been used for centuries to dye denim yarn.
The dyeing process is lengthy and begins as a white-green colour which turns blue once the fabric has been exposed to oxygen. The more often the fabric is dipped into the indigo bath, the deeper the blue becomes. Towards the end of the 19th century, Jean Baptiste Guimet developed a synthetic blue Indigo dye which eventually replaced the demand for traditionally produced dye.