Pink Horror vs Murderous Magenta

How close are they?

ΔE 15.0, loose: different enough that you should pick on purpose. Murderous Magenta is noticeably darker, more saturated than Pink Horror.

Can I use Murderous Magenta instead of Pink Horror?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Murderous Magenta is noticeably darker, more saturated than Pink Horror.

Can I use Pink Horror instead of Murderous Magenta?

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

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.