Fluor Yellow-Orange vs Green Rust

How close are they?

ΔE 63.5, loose: different enough that you should pick on purpose. Green Rust is a little lighter, much more muted, cooler than Fluor Yellow-Orange.

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

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 63). Green Rust is a little lighter, much more muted, cooler than Fluor Yellow-Orange.

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

The same difference applies the other way round: Fluor Yellow-Orange 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.