Warlock Magenta vs Nightmare Purple

How close are they?

ΔE 24.7, loose: different enough that you should pick on purpose. Nightmare Purple is noticeably darker, much more muted than Warlock Magenta.

Can I use Nightmare Purple instead of Warlock Magenta?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Nightmare Purple is noticeably darker, much more muted than Warlock Magenta.

Can I use Warlock Magenta instead of Nightmare Purple?

The same difference applies the other way round: Warlock Magenta 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.