One type of synaesthesia is music colour synaesthesia, where a note, key or certain quality in music causes the synaesthete to experience colour. Some music color synesthetes find the colours they see in music to be so distracting they can’t work while listening to music.
I find this fascinating because I don’t possess this kind of synaesthesia.
As a grapheme synaesthete, I do have colour responses to music but in a completely different way. For example, if I know the name of the song, then the song will be whichever colour I experience for the name (currently I’m listening to Sawdust and Diamonds by Joanna Newsom which is a lovely bright yellow for the whole 10 minutes of this epic song).
I am a very musical person (I play the piano, alto saxophone, surdo drum [brazilian] and a bit of clarinet, guitar and banjo) so in learning to read music I also had colour responses to keys and the letters of the notes if I knew which ones were being played, plus the instrument itself, because all instrument, like all words, have colours too.
Also, major and minor keys have an impact on the colours I perceive – minor keys generally add a black/grey/brown tinge to the letter of the key.
Although all this extra colour information is going on in my head it’s not overwhelming or distracting, it’s just there without intruding which is pretty lucky I guess.