I’m not technically synaestehtic but I do have strong associations between numbers and colors, days of the week and colors.
Saturday has always been blue. Sunday is red. Number 4 is green, 3 is yellow, etc.
For a time I moved to an Islamic country where Friday is the holy day, not Sunday. So the weekends were Thursday-Friday and not Saturday-Sunday.
What was weird is that my red/blue associations with Saturday/Sunday shifted onto Thursday/Friday after a long time of living like this. And then I left that place and my associations shifted back.
The “nineness” of a thing, as you say, is hard to describe.
