Fluorescent Orange vs Grim Rust

How close are they?

ΔE 30.6, loose: different enough that you should pick on purpose. Grim Rust is noticeably darker, much more muted, warmer than Fluorescent Orange.

Can I use Grim Rust instead of Fluorescent Orange?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 31). Grim Rust is noticeably darker, much more muted, warmer than Fluorescent Orange.

Can I use Fluorescent Orange instead of Grim Rust?

The same difference applies the other way round: Fluorescent Orange 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.