ΔE 18.6, loose: different enough that you should pick on purpose. Ulthuan Grey is much more muted than Neptune Glow.
Can I use Ulthuan Grey instead of Neptune Glow?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Ulthuan Grey is much more muted than Neptune Glow.
Can I use Neptune Glow instead of Ulthuan Grey?
The same difference applies the other way round: Neptune Glow is much 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.