There have been little tests performed on synesthesia, or how this condition is fundamentally forms. There are different assumptions of how people have developed this condition, some being a little outlandish. Some researchers have shown that people with synesthesia could just be "crazy," which I personally think is ridiculous. Another assumption is that as children, one may have played with numerical magnets. For example: "5" was red and "6" was blue... and that just stuck in their minds. Upon reading through the article, I believe that synesthesia is due to some cross wiring of the brain when a sensory system crosses over to another sensory system. I think there could be more research with synesthesia and the study of consciousness. Studying these perceptions may someday help us understand how we perceive the world.
Famous artists with Synesthesia:
... imagine how having synesthesia influenced their work.. interesting.