Pink Horror vs Hastur Purple

How close are they?

ΔE 24.2, loose: different enough that you should pick on purpose. Hastur Purple is noticeably darker, more muted, warmer than Pink Horror.

Can I use Hastur Purple instead of Pink Horror?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Hastur Purple is noticeably darker, more muted, warmer than Pink Horror.

Can I use Pink Horror instead of Hastur Purple?

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