Bullwhack Brown vs Walnut

How close are they?

ΔE 15.8, loose: different enough that you should pick on purpose. Walnut is a little darker, much more muted than Bullwhack Brown.

Can I use Walnut instead of Bullwhack Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Walnut is a little darker, much more muted than Bullwhack Brown.

Can I use Bullwhack Brown instead of Walnut?

The same difference applies the other way round: Bullwhack Brown is a little lighter, much 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.