Witchbane Plum vs Hastur Purple

How close are they?

ΔE 22.3, loose: different enough that you should pick on purpose. Hastur Purple is more muted, warmer than Witchbane Plum.

Can I use Hastur Purple instead of Witchbane Plum?

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

Can I use Witchbane Plum instead of Hastur Purple?

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