ΔE 9.1, close: similar, with a visible difference side by side. Grunge Brown is more saturated, warmer than Rust Orange.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Grunge Brown is more saturated, warmer than Rust Orange.
The same difference applies the other way round: Rust Orange is more muted, cooler.
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 Rust Orange on the colour wheel See Grunge Brown on the colour wheel
All matches: Rust Orange equivalents · Grunge Brown equivalents