ΔE 9.2, close: similar, with a visible difference side by side. Mechanicus Standard Grey is a little darker, more saturated than Slate Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Mechanicus Standard Grey is a little darker, more saturated than Slate Grey.
The same difference applies the other way round: Slate Grey is a little lighter, more muted.
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 Slate Grey on the colour wheel See Mechanicus Standard Grey on the colour wheel
All matches: Slate Grey equivalents · Mechanicus Standard Grey equivalents