I thought everyone had synesthesia
until I was eight. I felt I’d been very patient with everyone using the wrong colors, so I finally said something. My teacher told me there were no right or wrong colors for letters and numbers – we could all do what we wanted!!!!! I was miserable.
I have the most common type of synesthesia, color-graphemic, as well as some spatial/sequential weirdness. I took the synesthesia battery, which is strangely tiring, but informative.