ΔE 11.7, fair: the same colour family, but not a straight swap. Outlining Black is noticeably darker than Grim Black. They are different kinds of paint: Army Painter Grim Black is a one-coat contrast-style paint 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.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Outlining Black is noticeably darker than Grim Black. They are different kinds of paint.
The same difference applies the other way round: Grim Black is noticeably 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 Grim Black on the colour wheel See Outlining Black on the colour wheel
All matches: Grim Black equivalents · Outlining Black equivalents