ΔE 7.8, close: similar, with a visible difference side by side. Dark Rust is more muted than WWI French Brown.
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Dark Rust is more muted than WWI French Brown.
The same difference applies the other way round: WWI French Brown is more saturated.
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 WWI French Brown on the colour wheel See Dark Rust on the colour wheel
All matches: WWI French Brown equivalents · Dark Rust equivalents