ΔE 13.6, fair: the same colour family, but not a straight swap. Flat Red is more saturated, cooler than Rust.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Flat Red is more saturated, cooler than Rust.
The same difference applies the other way round: 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 Rust on the colour wheel See Flat Red on the colour wheel
All matches: Rust equivalents · Flat Red equivalents