ΔE 15.5, loose: different enough that you should pick on purpose. Leather White is a little lighter than Ulthuan Grey.
Can I use Leather White instead of Ulthuan Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Leather White is a little lighter than Ulthuan Grey.
Can I use Ulthuan Grey instead of Leather White?
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.