ΔE 13.3, fair: the same colour family, but not a straight swap. Moon Mist Grey is a little lighter than Ulthuan Grey.
Can I use Moon Mist 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). Moon Mist Grey is a little lighter than Ulthuan Grey.
Can I use Ulthuan Grey instead of Moon Mist Grey?
The same difference applies the other way round: Ulthuan Grey is a little darker.
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.