ΔE 13.1, fair: the same colour family, but not a straight swap. Cool Grey is noticeably darker than Ulthuan Grey.
Can I use Cool Grey instead of Ulthuan Grey?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Cool Grey is noticeably darker than Ulthuan Grey.
Can I use Ulthuan Grey instead of Cool Grey?
The same difference applies the other way round: Ulthuan Grey is noticeably lighter.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.