Witchbane Plum vs Plum

How close are they?

ΔE 17.8, loose: different enough that you should pick on purpose. Plum is noticeably lighter, cooler than Witchbane Plum.

Can I use Plum instead of Witchbane Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Plum is noticeably lighter, cooler than Witchbane Plum.

Can I use Witchbane Plum instead of Plum?

The same difference applies the other way round: Witchbane Plum is noticeably darker, warmer.

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.