Russian Blue Base vs Royal Purple

How close are they?

ΔE 8.5, close: similar, with a visible difference side by side. Royal Purple is a little lighter, warmer than Russian Blue Base.

Can I use Royal Purple instead of Russian Blue Base?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Royal Purple is a little lighter, warmer than Russian Blue Base.

Can I use Russian Blue Base instead of Royal Purple?

The same difference applies the other way round: Russian Blue Base is a little darker, 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.