ΔE 10.6, fair: the same colour family, but not a straight swap. Brown RLM26 is a little darker, cooler than Old Rust.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Brown RLM26 is a little darker, cooler than Old Rust.
The same difference applies the other way round: Old Rust is a little lighter, 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 Old Rust on the colour wheel See Brown RLM26 on the colour wheel
All matches: Old Rust equivalents · Brown RLM26 equivalents