I've just been reading a short chapter in one of Bryher's autobiographical novels, Development, called "The Colour of Words" - "It was impossible to think of the alphabet as colourless," the narrator writes.
Natural objects apart, which kept their actual hue, the initial letter gave the word its colour, but there were exceptions to this rule. Contrary to the French examples in the books she read, vowels were indecisive; it was the consonants that made a page as vivid as a sunset. Seven letters were white, C, G, Q, S, T, O, and U; three of the others were black, E, E, and I. W was crimson; H, M, and Y were various shades of gold and primrose. B changed from raspberry to umber, N was the rich tint of a red squirrel, F and J were a deeper brown. Other letters brought blue, as sharp as a broken wave, as dark as Alpine gentian. R was rose; A and P seemed too weak to be definite and varied with different words, though with names of places or people A was occasionally iris-blue or scarlet.
This has me excited to teach a course on color - an idea I have every couple of months, almost always when there's no possibility of choosing a new course offering or two even on the horizon. Synaesthesia would play only a small part in such a course, surely, but the passage would make for a good opening. What color do YOU see U? Hard, when I've just typed it in flaming crimson. But for me, undoubtedly, U is a warm, delicate dove grey.