ΔE 9.0, close: similar, with a visible difference side by side. Outlining Black is a little darker than Nuln Oil. They are different kinds of paint: Citadel Nuln Oil is a wash/shade and Nocturna Models Outlining Black is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Outlining Black is a little darker than Nuln Oil. They are different kinds of paint.
The same difference applies the other way round: Nuln Oil is a little lighter.
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 Outlining Black on the colour wheel
All matches: Nuln Oil equivalents · Outlining Black equivalents