Nabokov left the most literary account of grapheme-color synesthesia on record. In his memoir Speak, Memory he itemizes the colored alphabet: the long a of the English alphabet with the tint of weathered wood, the French a evoking polished ebony, letter by letter, in prose that doubles as data.
He described the trait as present from childhood, recounted disagreeing with his mother about the colors (she had them too, differently), and noted his wife Vera and son Dmitri were also synesthetes, an early literary record of the trait running in families.
His account is self-described but unusually rich and specific, and researchers cite it routinely as a canonical description of the grapheme form.