Synesthesia is when two senses combine with each other and form relationships. For example, I have the most common kind: grapheme-color synesthesia. Mine looks like this. The only one that’s supposed to be black is “1”. Otherwise, the ones in black have no color associated with them.
I don’t know to this day if I have Synesthesia, but some day as a small child I came to the conclusion that the numbers from 1 to 9 have “fitting” colours to them. It was somewhat based on the theory that the numbers that add to 10 have kind of “matching” colour combinations, but I can’t really remember any reasoning behind it.
These associations have stuck to me to this day, though.
Apparently for synesthetes, a few letters will likely have the same color association between people. I like the matching colors. :) So does zero not have a color for you?