ΔE 9.8, close: similar, with a visible difference side by side. Mechanicus Standard Grey is a little darker than Matt Ash Black. They are different kinds of paint: AMMO by Mig Matt Ash Black is an ink, glaze or transparent paint and Citadel Mechanicus Standard 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 10). Mechanicus Standard Grey is a little darker than Matt Ash Black. They are different kinds of paint.
The same difference applies the other way round: Matt Ash Black 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 Ash Black on the colour wheel See Mechanicus Standard Grey on the colour wheel
All matches: Matt Ash Black equivalents · Mechanicus Standard Grey equivalents