ΔE 14.6, fair: the same colour family, but not a straight swap. Ink Black Green is a little lighter, more saturated than Nuln Oil. They are different kinds of paint: Citadel Nuln Oil is a wash/shade and Vallejo Ink Black Green 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 15). Ink Black Green is a little lighter, more saturated than Nuln Oil. They are different kinds of paint.
The same difference applies the other way round: Nuln Oil is a little darker, 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 Nuln Oil on the colour wheel See Ink Black Green on the colour wheel
All matches: Nuln Oil equivalents · Ink Black Green equivalents