ΔE 14.3, fair: the same colour family, but not a straight swap. Ulthuan Grey is more muted than Marine Mist.
Can I use Ulthuan Grey instead of Marine Mist?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Ulthuan Grey is more muted than Marine Mist.
Can I use Marine Mist instead of Ulthuan Grey?
The same difference applies the other way round: Marine Mist is more saturated.
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.