ΔE 12.6, fair: the same colour family, but not a straight swap. Olive Ink is more saturated than Dark Tone. They are different kinds of paint: Army Painter Dark Tone is a wash/shade and Nocturna Models Olive 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 13). Olive Ink is more saturated than Dark Tone. They are different kinds of paint.
The same difference applies the other way round: Dark Tone 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 Dark Tone on the colour wheel See Olive Ink on the colour wheel
All matches: Dark Tone equivalents · Olive Ink equivalents