ΔE 4.4, very close: most people won’t see a difference side by side. Black is a little darker than Matt Black Primer. They are different kinds of paint: Army Painter Matt Black Primer is a technical paint or medium and Green Stuff World Black is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Yes, for most jobs. Very close match (ΔE 4). Black is a little darker than Matt Black Primer. They are different kinds of paint.
The same difference applies the other way round: Matt Black Primer 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 Matt Black Primer on the colour wheel See Black on the colour wheel
All matches: Matt Black Primer equivalents · Black equivalents