What color is the letter A? B? C? Some people hardly have to think and can immediately tell you the color of a letter, a number, or even a sound. This experience is called synesthesia, and for most people who experience it, it is an immensely pleasurable sensation. A sentence can take on a rich array of colors in the mind of the synesthete. The particular form of synesthesia that deals with color is called grapheme and is completely involuntary.
Dr. Richard Cytowic defines synesthesia with the five following factors:

1. Synesthetic images are spatially extended, meaning they often have a definite 'location'.
2. Synesthesia is involuntary and automatic.
3. Synesthetic percepts are consistent and generic (i.e. simple rather than imagistic).
4. Synesthesia is highly memorable.
5. Synesthesia is laden with affect.

Source: (Cytowic 2002, pp. 67-69; Cytowic 2003, pp. 76-77):

Many people to experience this phenomenon are able to utilize it in a creative and helpful way such as painting or as an aid for memorization. One woman recalls learning how to spell by remembering what color scheme it had. The prevalence of this phenomenon is unknown ranging everywhere from 1 in 20 to 1 in 20,000, but most researchers agree the cause is genetic. It is not uncommon to see a large percentage of synesthetes in families. Below is a link about a man who experiences synthesia : http://video.google.com/videoplay?docid=1079759211897095671&q=Synesthesia&total=444&start=0&num=10&so=0&type=search&plindex=0

Sources: http://en.wikipedia.org/wiki/Synesthesia
http://web.mit.edu/synesthesia/www/