ΔE 4.8, very close: most people won’t see a difference side by side. Matt Grey Primer is a little darker than Medium Grey. They are different kinds of paint: AMMO by Mig Medium Grey is an opaque base/layer paint and Army Painter Matt Grey Primer is a technical paint or medium, so they won’t behave the same on the model even where the colour is close.
Yes, for most jobs. Very close match (ΔE 5). Matt Grey Primer is a little darker than Medium Grey. They are different kinds of paint.
The same difference applies the other way round: Medium Grey 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 Medium Grey on the colour wheel See Matt Grey Primer on the colour wheel
All matches: Medium Grey equivalents · Matt Grey Primer equivalents