Witchbane Plum vs Xereus Purple

How close are they?

ΔE 21.7, loose: different enough that you should pick on purpose. Xereus Purple is more saturated, cooler than Witchbane Plum.

Can I use Xereus Purple instead of Witchbane Plum?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Xereus Purple is more saturated, cooler than Witchbane Plum.

Can I use Witchbane Plum instead of Xereus Purple?

The same difference applies the other way round: Witchbane Plum is more muted, 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.