ΔE 9.4, close: similar, with a visible difference side by side. Mechanicus Standard Grey is a little darker than RAF Extra Dark Sea Grey BS381C/640.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Mechanicus Standard Grey is a little darker than RAF Extra Dark Sea Grey BS381C/640.
The same difference applies the other way round: RAF Extra Dark Sea Grey BS381C/640 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 RAF Extra Dark Sea Grey BS381C/640 on the colour wheel See Mechanicus Standard Grey on the colour wheel
All matches: RAF Extra Dark Sea Grey BS381C/640 equivalents · Mechanicus Standard Grey equivalents