ΔE 10.3, fair: the same colour family, but not a straight swap. Light Brown is more saturated, warmer than Fresh Rust. They are different kinds of paint: Army Painter Fresh Rust is an opaque base/layer paint and Green Stuff World Light Brown is an ink, glaze or transparent paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Light Brown is more saturated, warmer than Fresh Rust. They are different kinds of paint.
The same difference applies the other way round: Fresh Rust is more muted, cooler.
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 Fresh Rust on the colour wheel See Light Brown on the colour wheel
All matches: Fresh Rust equivalents · Light Brown equivalents