ΔE 13.0, fair: the same colour family, but not a straight swap. Light Rust is much more saturated, warmer than Ground Brown.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Light Rust is much more saturated, warmer than Ground Brown.
The same difference applies the other way round: Ground Brown is much 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 Ground Brown on the colour wheel See Light Rust on the colour wheel
All matches: Ground Brown equivalents · Light Rust equivalents