Fluor Turquoise vs Green Rust

How close are they?

ΔE 28.3, loose: different enough that you should pick on purpose. Green Rust is noticeably lighter, more muted, warmer than Fluor Turquoise.

Can I use Green Rust instead of Fluor Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Green Rust is noticeably lighter, more muted, warmer than Fluor Turquoise.

Can I use Fluor Turquoise instead of Green Rust?

The same difference applies the other way round: Fluor Turquoise is noticeably darker, 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.