ΔE 15.2, loose: different enough that you should pick on purpose. Primer Dark Neutral Grey is much more muted than Mordant Earth.
Can I use Primer Dark Neutral Grey instead of Mordant Earth?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Primer Dark Neutral Grey is much more muted than Mordant Earth.
Can I use Mordant Earth instead of Primer Dark Neutral Grey?
The same difference applies the other way round: Mordant Earth 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.