ΔE 19.0, loose: different enough that you should pick on purpose. Chestnut Brown is noticeably lighter, more saturated, cooler than Dark Rust.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Chestnut Brown is noticeably lighter, more saturated, cooler than Dark Rust.
The same difference applies the other way round: Dark Rust is noticeably darker, 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 Chestnut Brown on the colour wheel
All matches: Dark Rust equivalents · Chestnut Brown equivalents