ΔE 8.9, close: similar, with a visible difference side by side. Infantry Blue is a little lighter, warmer than Russian Blue Base.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Infantry Blue is a little lighter, warmer than Russian Blue Base.
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.
See Russian Blue Base on the colour wheel See Infantry Blue on the colour wheel
All matches: Russian Blue Base equivalents · Infantry Blue equivalents