ΔE 7.3, close: similar, with a visible difference side by side. Plague Brown is more saturated than Mustard Yellow.
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Plague Brown is more saturated than Mustard Yellow.
The same difference applies the other way round: Mustard Yellow is 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.
See Mustard Yellow on the colour wheel See Plague Brown on the colour wheel
All matches: Mustard Yellow equivalents · Plague Brown equivalents