ΔE 10.9, fair: the same colour family, but not a straight swap. Mechanicus Standard Grey is a little darker than RLM 73.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Mechanicus Standard Grey is a little darker than RLM 73.
The same difference applies the other way round: RLM 73 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 RLM 73 on the colour wheel See Mechanicus Standard Grey on the colour wheel
All matches: RLM 73 equivalents · Mechanicus Standard Grey equivalents