Deep Violet vs Witchbane Plum

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Witchbane Plum is a little lighter, much more saturated, warmer than Deep Violet.

Can I use Witchbane Plum instead of Deep Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Witchbane Plum is a little lighter, much more saturated, warmer than Deep Violet.

Can I use Deep Violet instead of Witchbane Plum?

The same difference applies the other way round: Deep Violet is a little darker, much more muted, cooler.

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.