When is a spectrum not a spectrum?

There are 7 colours in the spectrum because of Newton’s book, Optics. He chose the 7 colours (red, orange, yellow, green, blue, indigo, violet) out of a belief that that there was a connection between the colours, the days of the week and the musical notes (proposed by Greek teachers). However, some claim there are in fact 6 colours in the spectrum due to the fact indigo and violet are so close to each other.