ΔE 16.9, loose: different enough that you should pick on purpose. Ghost White is noticeably lighter than Ulthuan Grey.
Can I use Ghost White instead of Ulthuan Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Ghost White is noticeably lighter than Ulthuan Grey.
Can I use Ulthuan Grey instead of Ghost White?
The same difference applies the other way round: Ulthuan Grey is noticeably 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.