Some people, such as the science writer Isaac Asimov (see Wikipedia), argue that indigo is not really a colour in its own right but merely a shade of blue.
Some people argue that indigo was invented simply to make the mnemonic (Richard Of York Gave Battle In Vain) work. This is contradicted by Wikipedia's version of the deliberations of another Isaac – Newton – who originally divided the spectrum into five colours (red, yellow, green, blue and violet) and later added orange and indigo.
Newton apparently chose to identify seven colours in order to comply with the ideas of the sophists (teachers of philosophy and related subjects) in ancient Greece, who thought there was a connection between the colours, the musical notes, the known objects in the Solar System, and the days of the week.
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