ΔE 8.7, close: similar, with a visible difference side by side. Mustard Brown is a little darker, more muted than Bullwhack Brown.
Usually, if you are not matching paint already on the model. Close match (ΔE 9). Mustard Brown is a little darker, more muted than Bullwhack Brown.
The same difference applies the other way round: Bullwhack Brown is a little lighter, more saturated.
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 Bullwhack Brown on the colour wheel See Mustard Brown on the colour wheel
All matches: Bullwhack Brown equivalents · Mustard Brown equivalents