ΔE 37.1, loose: different enough that you should pick on purpose. Fluor Red is a little lighter, much more saturated, cooler than Grim Rust.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 37). Fluor Red is a little lighter, much more saturated, cooler than Grim Rust.
The same difference applies the other way round: Grim Rust is a little darker, much 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 Grim Rust on the colour wheel See Fluor Red on the colour wheel
All matches: Grim Rust equivalents · Fluor Red equivalents