ΔE 8.6, close: similar, with a visible difference side by side. Dark Highlights is cooler than Mordant Earth. They are different kinds of paint: Citadel Mordant Earth is a technical paint or medium and Reaper Dark Highlights is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Dark Highlights is cooler than Mordant Earth. They are different kinds of paint.
The same difference applies the other way round: Mordant Earth is 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 Dark Highlights on the colour wheel
All matches: Mordant Earth equivalents · Dark Highlights equivalents