Russian Uniform vs Regiment Grey

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Our swatches for the two are almost the same.

Can I use Regiment Grey instead of Russian Uniform?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). The swatches are almost the same.

Can I use Russian Uniform instead of Regiment Grey?

The same difference applies the other way round.

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.