Here’s a short documentary about a group of people’s synesthetic experiences. It explains the phenomenon really clearly.
At 3:30 someone explains the subtleties of their color grapheme, and when I first saw this I was uber excited. For her, 1 is yellow, and 4 is more golden, which is the exact same for the way I see 2 (golden) and 8 (yellow). Getting giddy over something like that is pretty lame, I know, but I like to be able to relate to other synesthetes and think, “Oh, we both have golden letters. That’s so cool!” It’s a lil rare for me to hear someone with the exact same associations.