Fresh Rust vs Middle Brown

How close are they?

ΔE 7.4, close: similar, with a visible difference side by side. Middle Brown is a little darker, more saturated than Fresh Rust.

Can I use Middle Brown instead of Fresh Rust?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Middle Brown is a little darker, more saturated than Fresh Rust.

Can I use Fresh Rust instead of Middle Brown?

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

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.