ΔE 9.2, close: similar, with a visible difference side by side. Mars Black is a little darker than Mechanicus Standard Grey. They are different kinds of paint: Citadel Mechanicus Standard Grey is an opaque base/layer paint and Golden Mars Black is an ink, glaze or transparent 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 9). Mars Black is a little darker than Mechanicus Standard Grey. They are different kinds of paint.
The same difference applies the other way round: Mechanicus Standard 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 Mechanicus Standard Grey on the colour wheel See Mars Black on the colour wheel
All matches: Mechanicus Standard Grey equivalents · Mars Black equivalents