Mordant Earth vs Primer Black

How close are they?

ΔE 23.1, loose: different enough that you should pick on purpose. Primer Black is noticeably darker, much more muted than Mordant Earth.

Can I use Primer Black instead of Mordant Earth?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Primer Black is noticeably darker, much more muted than Mordant Earth.

Can I use Mordant Earth instead of Primer Black?

The same difference applies the other way round: Mordant Earth is noticeably lighter, 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.