ΔE 14.7, fair: the same colour family, but not a straight swap. Mechanicus Standard Grey is noticeably lighter, more muted than Russian Blue Lights.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 15). Mechanicus Standard Grey is noticeably lighter, more muted than Russian Blue Lights.
The same difference applies the other way round: Russian Blue Lights is noticeably darker, more saturated.
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 Russian Blue Lights on the colour wheel See Mechanicus Standard Grey on the colour wheel
All matches: Russian Blue Lights equivalents · Mechanicus Standard Grey equivalents