Some day , time or whatever the proper reference is, I am going to ask Sir Isaac Newton why he chose indigo as the seventh color in his spectrum. It is commonly believed that Newton wanted the spectrum to have seven colors because seven seemed to be the magic number for nature, and that is fine, but he had many other colors to choose from. Any of the other color blends would have worked, but he chose indigo!

I have a theory that Newton chose indigo because it was the color of the day. Until the seventeenth century Germany was the leading exporter of indigo dye, actually a blue dye from the woad plant. Then a series of events, including the thirty years war (1618-1648) and the introduction of indigo from the indigo plant in India led to the demise of the indigo trade in Germany.

The continental Europeans had banned the import of indigo for years because of its potential impact on the commercial production of woad. With the introduction of indigo by the British importers and the much richer color produced by the Indian indigo the ban soon had to be lifted.

So, Newton was probably aware of the activity with the import of the Indian indigo plant and whether consciously or subconsciously added indigo to his spectrum.