Pink Horror vs Light Magenta

How close are they?

ΔE 27.2, loose: different enough that you should pick on purpose. Light Magenta is noticeably lighter, more muted than Pink Horror.

Can I use Light Magenta instead of Pink Horror?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Light Magenta is noticeably lighter, more muted than Pink Horror.

Can I use Pink Horror instead of Light Magenta?

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