ΔE 7.8, close: similar, with a visible difference side by side. Outlining Black is a little darker than Dark Tone. They are different kinds of paint: Army Painter Dark Tone 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 8). Outlining Black is a little darker than Dark Tone. They are different kinds of paint.
The same difference applies the other way round: Dark Tone 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 Dark Tone on the colour wheel See Outlining Black on the colour wheel
All matches: Dark Tone equivalents · Outlining Black equivalents