Dark Wood vs Willow Bark

How close are they?

ΔE 20.1, loose: different enough that you should pick on purpose. Willow Bark is a little darker, much more muted than Dark Wood.

Can I use Willow Bark instead of Dark Wood?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Willow Bark is a little darker, much more muted than Dark Wood.

Can I use Dark Wood instead of Willow Bark?

The same difference applies the other way round: Dark Wood 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.