Fresh Rust vs Burnt Sienna

How close are they?

ΔE 8.0, close: similar, with a visible difference side by side. Burnt Sienna is a little darker, more muted than Fresh Rust.

Can I use Burnt Sienna instead of Fresh Rust?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Burnt Sienna is a little darker, more muted than Fresh Rust.

Can I use Fresh Rust instead of Burnt Sienna?

The same difference applies the other way round: Fresh Rust is a little lighter, 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.