I've been working on a piece about synaesthesia, which is, for those who don't know, a neurological condition that creates 'crossover' effects in experienced sensations. The most common form of this exists in those for whom letters and numbers have related (and specific) colors. There are musical variants that have colors related to certain keys or notes or even different chord progressions. I don't dare call it an 'affliction' because that has negative connotations and this does not sound negative in any way. In fact, it is reported that the vast majority of synaesthetes would hate to lose such a 'gift' and I could hardly disagree with such a statement. I'm actually kind of jealous, to be honest.
There are all sorts of artists who have this and probably loads of non-artist people, too. It is thought that 1 in 23 people have one of at least 54 different forms of the condition. I've mostly focused on musicians and composers who have it. Among them are Richard D. James (Aphex Twin), Duke Ellington, Eddie Van Halen (apparently dickheads can have it, too), Jean Sibelius, György Ligeti, Nikolai Rimsky-Korsakov and Olivier Messiaen. Below is a video (I don't know from when, but it looks a bit dated) of a Messiaen piece that is pretty rad. He saw different colors as keys changed, was a trained ornithologist and also a convert to Catholocism. I figure that's a pretty full life.