Wicked Pink vs Hastur Purple

How close are they?

ΔE 40.8, loose: different enough that you should pick on purpose. Hastur Purple is noticeably darker, much more muted, warmer than Wicked Pink.

Can I use Hastur Purple instead of Wicked Pink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 41). Hastur Purple is noticeably darker, much more muted, warmer than Wicked Pink.

Can I use Wicked Pink instead of Hastur Purple?

The same difference applies the other way round: Wicked Pink is noticeably lighter, much 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.