A quote from sycophantic review of Tammet's latest book by Simon Ings at a New Scientist magazine blog:
"Numbers have texture, colour and character. Whether we pay attention to these qualities, explore them and enjoy them, is a little bit to do with our genetic inheritance, much more to do with our schooling, and ultimately down to personal choice."
A genuine grapheme-colour and ordinal linguistic personification synaesthete says that is crap. Personified and coloured numbers or letters are completely subjective and idiosyncratic and untaught phenomena, which non-synaesthetes cannot experience in the same way as genuine, developmental synesthetes. Anyone who has told you different probably isn't a genuine synaesthete.