ΔE 9.0, close: similar, with a visible difference side by side. Light Rust is a little darker, more muted, warmer than Ruddy Umber.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Light Rust is a little darker, more muted, warmer than Ruddy Umber.
The same difference applies the other way round: Ruddy Umber is a little lighter, more saturated, 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 Ruddy Umber on the colour wheel See Light Rust on the colour wheel
All matches: Ruddy Umber equivalents · Light Rust equivalents