ΔE 7.5, close: similar, with a visible difference side by side. Fog Grey is a little lighter, more saturated, warmer than Matt Night Blue. They are different kinds of paint: AMMO by Mig Matt Night Blue is an ink, glaze or transparent paint and Army Painter Fog Grey 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 7). Fog Grey is a little lighter, more saturated, warmer than Matt Night Blue. They are different kinds of paint.
The same difference applies the other way round: Matt Night Blue is a little darker, more muted, cooler.
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 Night Blue on the colour wheel See Fog Grey on the colour wheel
All matches: Matt Night Blue equivalents · Fog Grey equivalents