Magenta vs Pink Horror

How close are they?

ΔE 18.6, loose: different enough that you should pick on purpose. Pink Horror is a little darker, more muted, cooler than Magenta.

Can I use Pink Horror instead of Magenta?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Pink Horror is a little darker, more muted, cooler than Magenta.

Can I use Magenta instead of Pink Horror?

The same difference applies the other way round: Magenta is a little lighter, more saturated, warmer.

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.