An outstanding thread.
I have always seen guitar cords as colours i.e.

E deep green
G light blue
A red
C yellow
D deep blue
F brown
B black

Don't know why but its always been there ever since I picked up a guitar.

I think its a personal thing depending on how the brain is wired up and probable how music relates to childhood experiences (maybe even pre-natal) There is an interesting line of enquiry regarding the Theosophical Society that was started around the turn of the last centaury by Madame Blavatsky. (1875) Several people wrote about the higher vibrations of the invisible world made manifest by material existence. ( Annie Besant and C. W. Leadbeater) Music is after all vibrations. Music and colour played a part in revealing these invisible vibrations. I think its in a book called 'thought forms' where different musical composers work can be seen through clairvoyant vision as coloured forms. Just another angle to this fascinating thread.