It's definately Synesthesia
http://en.wikipedia.org/wiki/Grapheme-color_synesthesia
http://news.bbc.co.uk/2/hi/health/4375977.stm
http://en.wikipedia.org/wiki/Synesthesia
A bunch of musicians, artists, and writers have Synesthesia. I remember reading a study saying that these types of people had a higher rate of Synesthesia than the general population. Pharrel Williams is a pretty famous person who hears sounds as colours.