Newton originally (1672) divided the spectrum into five main colours: red, yellow, green, blue and violet.
Later he included orange and indigo, giving seven main colours by analogy to the number of notes in a musical scale.[2][9] Newton chose to divide the visible spectrum into seven colours out of a belief derived from the beliefs of the ancient Greek sophists, 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.[10][11][12]