ΔE 12.5, fair: the same colour family, but not a straight swap. Russ Grey is a little darker, more saturated than Space Grey.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Russ Grey is a little darker, more saturated than Space Grey.
The same difference applies the other way round: Space Grey is a little lighter, more muted.
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 Space Grey on the colour wheel See Russ Grey on the colour wheel
All matches: Space Grey equivalents · Russ Grey equivalents