US Army Helo Drab FS 34031 vs NATO Brown

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Our swatches for the two are almost the same.

Can I use NATO Brown instead of US Army Helo Drab FS 34031?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). The swatches are almost the same.

Can I use US Army Helo Drab FS 34031 instead of NATO Brown?

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.