David Eagleman is coming on Tuesday of this week to give a lecture, among the many things he writes about synesthesia is one. Synesthesia is the autonomic pairing of senses, a rare phenomena occurring in 1 in everyone 20-25,000 people (the current estimate is that about 1 in 23 have some light form). These correlations are most commonly seen in people who pair a letter or number to a color but cases have been reported where sounds was paired to colors, a numerical order (day of the week or the time of day) paired with personalities, or even the pairing of words with tastes. Eagleman has wrote a book entitled Wednesday is Indigo Blue and if you are interested it is available through ebrary academic complete through the library portal.
Neurologically speaking the V4 area of the brain is generally given credit for visual synesthesia but most believe it is not solely due to this area alone. Others believe the phenomena is caused by synapses not trimming enough at young ages, so the body correlates two things together that don't necessarily belong together. However, due to the large variety of synesthesia that exists it is likely we are really dealing with several different neurological happening (depending on the case) and giving them an umbrella term to function under. Genetically all forms of synesthesia are believed to be linked to a currently unknown gene as it does run in families. The gene was originally thought to linked to the X as it could skip generations but recent data has also shown cases of father to son transmission. Basically, a whole lot is known about the condition but a lot more could be known.