Feynman’s account is brief, firsthand, and often quoted: he described seeing colored symbols when working through equations, light tan js, slightly violet-bluish ns, dark brown xs, and mused about what the equations could look like to students who presumably saw them colorless.
The report appears in his own published recollections, which makes it self-described despite its brevity. It is a clean example of grapheme-color synesthesia operating on mathematical notation rather than prose, a reminder that the inducer is the learned symbol system, whichever one a mind happens to run on.
Feynman’s case is popular on lists because of who he was; it earns its place because the words are his.