ΔE 12.8, fair: the same colour family, but not a straight swap. Olive Ink is a little darker, much more saturated than Black Templar. They are different kinds of paint: Citadel Black Templar 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 13). Olive Ink is a little darker, much more saturated than Black Templar. They are different kinds of paint.
The same difference applies the other way round: Black Templar is a little lighter, 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 Templar on the colour wheel See Olive Ink on the colour wheel
All matches: Black Templar equivalents · Olive Ink equivalents