This condition has always held some fascination for me ever since I knew of Messiaen's constant correlation of notes/chords with colours. I also recently saw a doco on TV which I wish I'd been able to record as it was about Synesthesia in it's many forms.
From what I can gather though, the condition varies dramatically from person to person in that there is no constant - scientifically speaking anyway. I mean theoretically, it's possible to draw a correlation chart between the frequency of coloured light and the frequency of musical pitch, but if you're measuring in Hertz, the visible light range is inaudible, so not even physics can tell us, for example, that a G# is blue (or something like that). There's certainly no scientific constant to tell us the relationship between a letter and a colour, which is another form of synesthesia.
Messiaen's synesthesia was quite individual, because he would often see combinations of colours, and in differing patterns too. For example, one of his favourite chords from his second mode of limited transposition (aka the octotonic scale) he would describe as being basically purple, but with occasional waves of orange, and with gold flecks (or spots)!
On the other hand, Scriabin's synesthesia (or 'alleged' synesthesia as some people claim) was completely different - he saw plain shades of colours, which led him to the composition of a number of works directly related to colour. It also led to his development of the 'colour organ' (an instrument which in turn, apparently inspired the large screen that was hooked up to the ARP 2500 for the musical conversation in Close Encounters Of The Third Kind)
I think also that Liszt and Rimsky-Korsakov were supposed to have had it in varying degrees, along with Duke Ellington, and I think I'm recalling guitarist Allan Holdsworth in relation to it as well but I could be getting way off the mark and confused already. It's a fascinating topic and one which makes it clear that the brain still has countless untapped wonders.