Here, Vladimir Nabokov, 1 of the most famous synaesthetes, described his experience:
"…I present a fine case of colored hearing. Perhaps “hearing” is not quite accurate, since the color sensations seem to be produced by the very act of my orally forming a given letter while I imagine its outline. The long a of the English alphabet (and it is this alphabet I have in mind farther on unless otherwise stated) has for me the tint of weathered wood, but the French a evokes polished ebony. This black group also includes hard g (vulcanized rubber) and r (a sooty rag being ripped). Oatmeal n, noodle-limp l, and the ivory-backed hand mirror of o take care of the whites. I am puzzled by my French on which I see as the brimming tension-surface of alcohol in a small glass. Passing on to the blue group, there is steely x, thundercloud z, and hucklberry k. Since a subtle interaction exists between sound and shape, I see q as browner than k, while s is not the light blue of c, but a curious mixture of azure and mother-of-pearl. Adjacent tints do not merge, and dipthongs do not have special colors unless represented by a single character in some other language (thus the fluffy-gray, three-stemmed Russian letter that stands for sh, a letter as old as the rushes of the Nile, influences its English representation).I hasten to complete this list before I am interrupted. In the green group, there are alder-leaf f, the unripe apple of p, and pistachio t. Dull green, combined somehow with violet, is the best I can do for w. The yellows comprise various e’s and i’s, creamy d, bright-golden y, and u, whose alphabetical value I can express only by “brassy with an olive sheen.” In the brown group, there are the rich rubbery tone of soft g, paler j, and the drab shoelace of h. Finally, among the reds, b has the tone called burnt sienna by painters, m is a fold of pink flannel, and today I have at last perfectly matched v with “Rose Quartz” in Maerz and Paul’s Dictionary of Color. The word for rainbow, a primary, but decidedly muddy, rainbow is in my private language the hardly pronouncable: kzspygv. The first author to discuss audition coloreé was, as far as I know, an albino physician in 1812, in Erlangen.The confessions of a synesthete must sound tedious and pretentious to those who are protected from such leakings and drafts by more solid walls than mine are. To my mother, though, this all seemed quite normal. The matter came up, one day in my seventh year, as I was using a head of old alphabet blcks to build a tower. I casually remarked to her that the colors were all wrong. We discovered that some of her letters had the same tint as mine, and that, besides, she was optically affected by musical notes. These evoked no chromatisms in me whatsoever. Music, I regret to say, affects me merely as an arbitrary succession of more ore less irritating sounds. Under certain circumstances I can stand the spasms of a rich violin, but the concern piano and wind instruments bore me in small doses and flay me in large ones."
[1 blogger even tried to reproduce what Nabokov saw in his mind:
More interestingly, he 'heard' different colours in different languages:
"The long "a" of the English alphabet has for me the tint of weathered wood, but a French "a" evokes polished ebony. This black group also includes hard "g" (vulcanized rubber) and "r" (a sooty rag being ripped). Oatmeal "n", noodle-limp "l", and the ivory-backed hand mirror of an "o" take care of the whites. I am puzzled by my French "on" which I see as the brimming tension-surface of alcohol in a small glass."
Even his wife Vera and his son had synaesthesia:
"My wife has this gift of seeing letters in color, too, but her colors are completely different. There are, perhaps, two or three letters where we coincide, but otherwise the colors are quite different. It turned out, we discovered one day, that my son, who was a little boy at the time—I think he was ten or eleven—sees letters in colors, too. Quite naturally he would say, "Oh, this isn't that color, this is this color," and so on. Then we asked him to list his colors and we discovered that in one case, one letter which he sees as purple, or perhaps mauve, is pink to me and blue to my wife. This is the letter M. So the combination of pink and blue makes lilac in his case. Which is as if genes were painting in aquarelle."(source: mentalfloss)
I never knew about that combination of colours- isn't that amazing?