ΔE 10.4, fair: the same colour family, but not a straight swap. Brown Ink is more saturated than Mordant Earth. They are different kinds of paint: Citadel Mordant Earth is a technical paint or medium and Reaper Brown Ink is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Brown Ink is more saturated than Mordant Earth. They are different kinds of paint.
The same difference applies the other way round: Mordant Earth is 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 Mordant Earth on the colour wheel See Brown Ink on the colour wheel
All matches: Mordant Earth equivalents · Brown Ink equivalents