ΔE 12.6, fair: the same colour family, but not a straight swap. Willow Bark is noticeably darker than Green Stone.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Willow Bark is noticeably darker than Green Stone.
The same difference applies the other way round: Green Stone is noticeably lighter.
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 Green Stone on the colour wheel See Willow Bark on the colour wheel
All matches: Green Stone equivalents · Willow Bark equivalents