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.
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?