ΔE 13.2, fair: the same colour family, but not a straight swap. Purple Black is more muted than Agrax Earthshade. They are different kinds of paint: Citadel Agrax Earthshade is a wash/shade and Nocturna Models Purple 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 13). Purple Black is more muted than Agrax Earthshade. They are different kinds of paint.
The same difference applies the other way round: Agrax Earthshade is more saturated.
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 Agrax Earthshade on the colour wheel See Purple Black on the colour wheel
All matches: Agrax Earthshade equivalents · Purple Black equivalents