Red Ecstasy vs Rust

How close are they?

ΔE 68.7, loose: different enough that you should pick on purpose. Rust is noticeably darker, much more muted than Red Ecstasy.

Can I use Rust instead of Red Ecstasy?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 69). Rust is noticeably darker, much more muted than Red Ecstasy.

Can I use Red Ecstasy instead of Rust?

The same difference applies the other way round: Red Ecstasy is noticeably lighter, much more saturated.

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.