ΔE 12.3, fair: the same colour family, but not a straight swap. Olive Ink is much more saturated than Black Legion. They are different kinds of paint: Citadel Black Legion is a one-coat contrast-style paint 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 12). Olive Ink is much more saturated than Black Legion. They are different kinds of paint.
The same difference applies the other way round: Black Legion is much 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 Black Legion on the colour wheel See Olive Ink on the colour wheel
All matches: Black Legion equivalents · Olive Ink equivalents