"No two people agree, or hardly ever do so, as to the colour they associates with the same sound." - Harrison J. (2001). Synaesthesia. Oxford: Oxford University Press.
Yet somehow, three synesthetes can agree on four letters: A (red), R (red), S (yellow) and W (dark blue). Throughout my research, and speaking to others with synesthesia, I have come across multiple examples of A being depicted as red. It seems that the most common colour association is that of A being red.