>I don't have anything other than personal experience to back this up, but I would be surprised if it's not a completely subjective experience—one person's synesthesia probably doesn't match another person's, per se.

From what I recall from the literature (it was a while ago, so details elude me), that while there does seem to be some relatively common experiences (many people associating a particular number with a particular colour), there's definitely a lot that's very idiosyncratic. I recall some people speculating that some of these associations could be shared because of common books/teaching materials, that maybe many children grew up with the same books showing letters and numbers in various colours. But that's just speculation as far as I'm aware.