Nightmare Purple vs Magenta

How close are they?

ΔE 27.0, loose: different enough that you should pick on purpose. Magenta is noticeably lighter, more saturated, warmer than Nightmare Purple.

Can I use Magenta instead of Nightmare Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Magenta is noticeably lighter, more saturated, warmer than Nightmare Purple.

Can I use Nightmare Purple instead of Magenta?

The same difference applies the other way round: Nightmare Purple is noticeably darker, 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.