ΔE 38.9, loose: different enough that you should pick on purpose. Mordant Earth is noticeably darker, more saturated than Matt Grey Primer.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 39). Mordant Earth is noticeably darker, more saturated than Matt Grey Primer.
The same difference applies the other way round: Matt Grey Primer is noticeably lighter, more muted.
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.
See Matt Grey Primer on the colour wheel See Mordant Earth on the colour wheel
All matches: Matt Grey Primer equivalents · Mordant Earth equivalents