Grim Rust vs Fluor Yellow-Orange

How close are they?

ΔE 27.0, loose: different enough that you should pick on purpose. Fluor Yellow-Orange is noticeably lighter, more muted than Grim Rust.

Can I use Fluor Yellow-Orange instead of Grim Rust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Fluor Yellow-Orange is noticeably lighter, more muted than Grim Rust.

Can I use Grim Rust instead of Fluor Yellow-Orange?

The same difference applies the other way round: Grim Rust is noticeably 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.