ΔE 8.7, close: similar, with a visible difference side by side. Red Brown is more saturated, cooler than Dark Rust.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Red Brown is more saturated, cooler than Dark Rust.
The same difference applies the other way round: Dark Rust is more muted, warmer.
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 Dark Rust on the colour wheel See Red Brown on the colour wheel
All matches: Dark Rust equivalents · Red Brown equivalents