We discoverd our ds saw numbers as colors two years ago when he was nine. He also experiences feelings as tastes and colors. It explained sooooo much!!! He is very intense and sensitive and now I know why! It explained why a kid who could hear a song once and have it memorized or read something once and have it memorized, couldn't do mulitiplication. It especially came out when they had timed mulitiplicationt tests in school. If he had time he could figure it out but the timed tests really threw him. He is in a public charter school that is an open classroom whole child education type place so when he needs to separate himself because of sensory overload he is able to.
An interesting side note: He sees number nine as black. When I was explaining to his sister, who is 20 years older than him (different dad,) about his synesthesia and how he sees numbers as colors she responded with "Uh huh, so?" Seems she has always seen numbers as colors and nine is also black! Explained sooooo much about her, also!
It has become less of an issue as our ds gets closer to puberty and mostly shows up in times of stress. Same for my daughter. So interesting!!