ΔE 8.8, close: similar, with a visible difference side by side. Fresh Rust is a little darker, cooler than Mushroom Brown.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Fresh Rust is a little darker, cooler than Mushroom Brown.
The same difference applies the other way round: Mushroom Brown is a little lighter, 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.
See Mushroom Brown on the colour wheel See Fresh Rust on the colour wheel
All matches: Mushroom Brown equivalents · Fresh Rust equivalents