Very good question asked on Colour Chat blog, and a partial answer, at the link below:

Post by @stephenwestland about the place of indigo in the rainbow. Should it be there? #visualization #color #science http://t.co/CKPBi3ME

— Matteo Niccoli (he/him) (@My_Carta) November 14, 2011

Also:

George Biernson, in 1972, wrote in the American Journal of Physics Why Did Newton See Indigo in the Spectrum? and “hypothesizes that Newton saw seven reasonably distinct colors in the artist’s paint mixture color circle (red, orange, yellow, green, blue, violet, and purple) and therefore assumed he could also see seven distinct colors in his crude spectral projections”.

Others have argued Newton was trying to add a seventh color to match the seven notes of the western world’s musical scale.

I will tackle the many problems of rainbow in my forthcoming series The rainbow is dead…long live the rainbow!!!

Stay tuned…