India is believed to be the oldest center of indigo dyeing in the Old World. It was a primary supplier of indigo dye, derived from the plant Indigofera tinctoria, to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the ‘dye’, which was indikon (ινδικόν). The Romans used the term indicum, which passed into Italian dialect and eventually into English as the word indigo. El Salvador has lately been the biggest producer of indigo.
The same indigo dye is contained in the woad plant, Isatis tinctoria, for a long time the main source of blue dye in Europe. Woad was replaced by true indigo as trade routes opened up, and both are now largely replaced by synthetic dyes.
Though the word indigo has existed in English since the 13th century, it may never have been a common part of the basic color-naming system.