ΔE 15.5, loose: different enough that you should pick on purpose. Ulthuan Grey is more muted than Pale Grey (RLM 76).
Can I use Ulthuan Grey instead of Pale Grey (RLM 76)?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ulthuan Grey is more muted than Pale Grey (RLM 76).
Can I use Pale Grey (RLM 76) instead of Ulthuan Grey?
The same difference applies the other way round: Pale Grey (RLM 76) 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.