Pink Horror vs Pale Violet Red

How close are they?

ΔE 19.2, loose: different enough that you should pick on purpose. Pale Violet Red is noticeably darker, much more muted than Pink Horror.

Can I use Pale Violet Red instead of Pink Horror?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Pale Violet Red is noticeably darker, much more muted than Pink Horror.

Can I use Pink Horror instead of Pale Violet Red?

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

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.