ΔE 75.5, loose: different enough that you should pick on purpose. Fluor White is noticeably lighter, much more muted than Grim Rust.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 75). Fluor White is noticeably lighter, much more muted than Grim Rust.
The same difference applies the other way round: Grim Rust is noticeably darker, much 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 Grim Rust on the colour wheel See Fluor White on the colour wheel
All matches: Grim Rust equivalents · Fluor White equivalents