9 Colors is a bit limiting, as you get red to purple with like 6, and then brown, grey, and white take up the next 3, leaving little room for middle-colors like pink or cyan, or having 2 shades like lime and dark green.
I think having 18 colors would be easily do-able, as you could use Shift with 1-9 to give you 18. Also 0 should be in the number list too, meaning you’d get 20.
TL;DR: Shift + 1-9 for 18 colors, and adding 0 giving you 2 more, so 20 colors.