ΔE 14.0, fair: the same colour family, but not a straight swap. Slate Grey is a little lighter, more muted than Dark Rust.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Slate Grey is a little lighter, more muted than Dark Rust.
The same difference applies the other way round: Dark Rust is a little darker, more saturated.
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 Slate Grey on the colour wheel
All matches: Dark Rust equivalents · Slate Grey equivalents