Pink Horror vs Primary Magenta

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Primary Magenta is a little darker, more saturated, warmer than Pink Horror.

Can I use Primary Magenta instead of Pink Horror?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Primary Magenta is a little darker, more saturated, warmer than Pink Horror.

Can I use Pink Horror instead of Primary Magenta?

The same difference applies the other way round: Pink Horror is a little lighter, more muted, 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.