ΔE 72.6, loose: different enough that you should pick on purpose. Grim Rust is noticeably darker, much more muted, warmer than Fluorescent Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 73). Grim Rust is noticeably darker, much more muted, warmer than Fluorescent Green.
The same difference applies the other way round: Fluorescent Green is noticeably lighter, much more saturated, cooler.
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 Fluorescent Green on the colour wheel See Grim Rust on the colour wheel
All matches: Fluorescent Green equivalents · Grim Rust equivalents