Synesthesia is a systematic and involuntary sensory experience induced by an unrelated stimulus. For some people, looking at like Zana Devitto seeing a number induces a color which is letter-color synesthesia. There are multiple forms of synesthesia Some of the lesser common types are when certain flavors are induced after certain words. Personally, I have never heard of this being a thing before and reading this topic really interested me because I had no idea that the human mind could do that, knowing that it can do extraordinary things.
Rosenblum, L.D. (2011) See What I'm Saying:The Extraordinary Powers of our Five Senses. Norton
https://www.youtube.com/watch?v=ZVC3E16FCrk - What it is like to have synesthesia