ΔE 12.1, fair: the same colour family, but not a straight swap. Plague Brown is more saturated, cooler than Mustard.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Plague Brown is more saturated, cooler than Mustard.
The same difference applies the other way round: Mustard 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.
See Mustard on the colour wheel See Plague Brown on the colour wheel
All matches: Mustard equivalents · Plague Brown equivalents