From “The Poet Who Could Smell Vowels,” an arti­cle by John E. Joseph in the cur­rent TLS that quotes Fer­di­nand de Saussure’s record of his synes­the­sia (first included anony­mously in a study by William James’s friend Théodore Flournoy):

In French we write the same vowel four dif­fer­ent ways in ter­rain, plein, matin, chien. Now when this vowel is writ­ten ain, I see it in pale yellow like an incom­pletely baked brick; when it is writ­ten ein, it strikes me as a net­work of pur­plish veins; when it is writ­ten in, I no longer know at all what colour sen­sa­tion it evokes in my mind, and am inclined to believe that it evokes none….

So it does not seem to be the vowel as such—as it exists for the ear, that is—that calls forth a cer­tain cor­re­spond­ing visual sen­sa­tion. On the other hand, nei­ther is it seeing a cer­tain letter or group of let­ters that calls forth this sen­sa­tion. Rather it is the vowel as it is con­tained in this writ­ten expres­sion, it is the imag­i­nary being formed by this first asso­ci­a­tion of ideas which, through another asso­ci­a­tion, appears to me as endowed with a cer­tain con­sis­tency and a cer­tain colour, some­times also a cer­tain shape and a cer­tain smell.

Joseph also quotes Saussure’s expe­ri­ence of the French letter-​sound a:

[It is] off-​white, approach­ing yellow; in its con­sis­tency, it is some­thing solid, but thin, that cracks easily if struck, for exam­ple a sheet of paper (yel­lowed with age) drawn tight in a frame, a flimsy door (in unvar­nished wood left white) that you feel would shat­ter at the slight­est blow, an already broken eggshell that you can keep crack­ing by press­ing on it with your fin­gers. Better still: the shell of a raw egg is a (whether in colour or in the con­sis­tency of the object), but the shell of a hard-​boiled egg is not a, because of the feel­ing you have that the object is com­pact and resis­tant. A yel­lowed pane of glass is a; a pane of ordi­nary colour, offer­ing blueish reflec­tions, is the very oppo­site of a, because of its colour, and despite its con­sis­tency being just right.