Rust vs Dreadnought Yellow

How close are they?

ΔE 26.2, loose: different enough that you should pick on purpose. Dreadnought Yellow is noticeably lighter, more saturated, warmer than Rust.

Can I use Dreadnought Yellow instead of Rust?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Dreadnought Yellow is noticeably lighter, more saturated, warmer than Rust.

Can I use Rust instead of Dreadnought Yellow?

The same difference applies the other way round: Rust is noticeably darker, more muted, 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.