ΔE 6.2, close: similar, with a visible difference side by side. Olive Brown is a little lighter than Barrel Grey.
Usually, if you are not matching paint already on the model. Close match (ΔE 6). Olive Brown is a little lighter than Barrel Grey.
The same difference applies the other way round: Barrel Grey is a little darker.
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 Barrel Grey on the colour wheel See Olive Brown on the colour wheel
All matches: Barrel Grey equivalents · Olive Brown equivalents