Fluorescent Green vs Green Rust

How close are they?

ΔE 52.1, loose: different enough that you should pick on purpose. Green Rust is much more muted, cooler than Fluorescent Green.

Can I use Green Rust instead of Fluorescent Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 52). Green Rust is much more muted, cooler than Fluorescent Green.

Can I use Fluorescent Green instead of Green Rust?

The same difference applies the other way round: Fluorescent Green is much more saturated, warmer.

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.