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
Waiting for your follow up!
Should be ready to start that series before the end of February. Thanks
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For Indigo, I do see a separate distinguishable color between blue and violet. The author is right about the mixture of the colors (wavelengths) can give the secondary colors. But in the eye each of the different types of cones, 3 of them though some says some women has 4 different cones, has a different response as one comes vary the wavelength. The cones down through blue wavelengths has definite peaks in the response to colors. The Red cone is of course centered in the “Red” wavelength, the Yellow in the “Yellow” wavelength and the Blue in the blue wavelength. To make orange that wavelength excites the red and yellow cones, green excites the yellow and blue cones. But violet excites the red and blue but not the green cone. The wavelength filters for the green prevents that. Now I don’t know the response of each type of cone to indigo light but It could have excite all 3 cones. There is good physical reasons about this as electrons are excited from a molecular valence band to the vacuum creating a free electron that can be used by the brain for the color.
Now one can get purple from shining a blue and red light separately but one needs to adjust the relative intensity between them to get the violet one sees in the rainbow. That goes for all the other secondary colors.
Because some people can see the indigo band and some may not be able to see it, that can be because of the difference in response between the cones people has. There isn’t any reason the filters between people need to be the same.
So do we limit the colors definition to the lowest common denominator or the greatest?