ΔE 11.8, fair: the same colour family, but not a straight swap. Mechanicus Standard Grey is noticeably lighter than Raven Black.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Mechanicus Standard Grey is noticeably lighter than Raven Black.
The same difference applies the other way round: Raven Black is noticeably 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 Raven Black on the colour wheel See Mechanicus Standard Grey on the colour wheel
All matches: Raven Black equivalents · Mechanicus Standard Grey equivalents