Synaesthesia is very personal.  I experience tones, keys, chords also
as colours but I'll never be a Messiaen I'm afraid.  I have the same
thing with numbers and the days of the week - so maybe I should keep
a diary ;) The most interesting book I ever read on the subject in its
extreme and pathological form is by the Russian neurologist (and Oliver
Sachs-predecessor and inspiration) Luria.  It's called *The Mind of a
Menomist*.  I am off topic here of course but the book *is* something else.

Philip