Bullwhack Brown vs Raw Sienna

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Raw Sienna is noticeably darker, more saturated than Bullwhack Brown.

Can I use Raw Sienna instead of Bullwhack Brown?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Raw Sienna is noticeably darker, more saturated than Bullwhack Brown.

Can I use Bullwhack Brown instead of Raw Sienna?

The same difference applies the other way round: Bullwhack Brown is noticeably lighter, 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.