ΔE 15.8, loose: different enough that you should pick on purpose. Ink Skin is noticeably darker, cooler than Mordant Earth. They are different kinds of paint: Citadel Mordant Earth is a technical paint or medium and Vallejo Ink Skin is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ink Skin is noticeably darker, cooler than Mordant Earth. They are different kinds of paint.
The same difference applies the other way round: Mordant Earth is noticeably lighter, 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 Ink Skin on the colour wheel
All matches: Mordant Earth equivalents · Ink Skin equivalents