ΔE 10.1, fair: the same colour family, but not a straight swap. Buff is a little lighter, cooler than Mustard Yellow.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). Buff is a little lighter, cooler than Mustard Yellow.
The same difference applies the other way round: Mustard Yellow is a little darker, 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 Yellow on the colour wheel See Buff on the colour wheel
All matches: Mustard Yellow equivalents · Buff equivalents