A very interesting article via nytimes.com: Brainman, at Rest in His Oasis
My favorite part:
But he still speaks of numbers with affection and says that for him they have individual personalities, particularly the smaller ones. The number four, for instance, is shy, and reminds him of himself; nines are scary and imposing. Ones are shiny and bright, eights are blue, fives are loud, and 333 is beautiful.
Giving personality and colors to numbers is a form of synesthesia. Just another reminder to me about how amazing the brain really is.
How someone with synesthesia might perceive (not "see") certain letters and numbers.