Fluor Lime vs Green Rust

How close are they?

ΔE 35.5, loose: different enough that you should pick on purpose. Green Rust is a little lighter, much more muted, cooler than Fluor Lime.

Can I use Green Rust instead of Fluor Lime?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 35). Green Rust is a little lighter, much more muted, cooler than Fluor Lime.

Can I use Fluor Lime instead of Green Rust?

The same difference applies the other way round: Fluor Lime is a little darker, 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.