Orange Brown vs Fresh Rust

How close are they?

ΔE 7.6, close: similar, with a visible difference side by side. Fresh Rust is more saturated, cooler than Orange Brown.

Can I use Fresh Rust instead of Orange Brown?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Fresh Rust is more saturated, cooler than Orange Brown.

Can I use Orange Brown instead of Fresh Rust?

The same difference applies the other way round: Orange Brown 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.