ΔE 8.5, close: similar, with a visible difference side by side. Stormy Grey is a little lighter than Mechanicus Standard Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Stormy Grey is a little lighter than Mechanicus Standard Grey.
The same difference applies the other way round: Mechanicus Standard Grey is a little darker.
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 Stormy Grey on the colour wheel
All matches: Mechanicus Standard Grey equivalents · Stormy Grey equivalents