Fluor Blue vs Rust

How close are they?

ΔE 46.6, loose: different enough that you should pick on purpose. Rust is noticeably darker, much more muted, warmer than Fluor Blue.

Can I use Rust instead of Fluor Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 47). Rust is noticeably darker, much more muted, warmer than Fluor Blue.

Can I use Fluor Blue instead of Rust?

The same difference applies the other way round: Fluor Blue 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.