ΔE 9.4, close: similar, with a visible difference side by side. Sombre Grey is a little lighter, more muted, warmer than Russ Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Sombre Grey is a little lighter, more muted, warmer than Russ Grey.
The same difference applies the other way round: Russ Grey is a little darker, more saturated, cooler.
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 Russ Grey on the colour wheel See Sombre Grey on the colour wheel
All matches: Russ Grey equivalents · Sombre Grey equivalents