Fluor Turquoise vs Rust

How close are they?

ΔE 45.1, loose: different enough that you should pick on purpose. Rust is noticeably darker, much more muted, warmer than Fluor Turquoise.

Can I use Rust instead of Fluor Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 45). Rust is noticeably darker, much more muted, warmer than Fluor Turquoise.

Can I use Fluor Turquoise instead of Rust?

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