ΔE 41.7, loose: different enough that you should pick on purpose. Primer Dark Purple is a little darker, much more saturated, cooler than Mordant Earth.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 42). Primer Dark Purple is a little darker, much more saturated, cooler than Mordant Earth.
The same difference applies the other way round: Mordant Earth is a little lighter, much more muted, warmer.
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 Mordant Earth on the colour wheel See Primer Dark Purple on the colour wheel
All matches: Mordant Earth equivalents · Primer Dark Purple equivalents