Russian Blue vs Deep Grey

How close are they?

ΔE 9.3, close: similar, with a visible difference side by side. Deep Grey is more muted, warmer than Russian Blue.

Can I use Deep Grey instead of Russian Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Deep Grey is more muted, warmer than Russian Blue.

Can I use Russian Blue instead of Deep Grey?

The same difference applies the other way round: Russian Blue is 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.