Dark Rust vs Russet Brown

How close are they?

ΔE 6.7, close: similar, with a visible difference side by side. Russet Brown is more saturated, cooler than Dark Rust.

Can I use Russet Brown instead of Dark Rust?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Russet Brown is more saturated, cooler than Dark Rust.

Can I use Dark Rust instead of Russet Brown?

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