ΔE 19.9, loose: different enough that you should pick on purpose. Deep Magenta is noticeably lighter, warmer than Nightmare Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Deep Magenta is noticeably lighter, warmer than Nightmare Purple.
The same difference applies the other way round: Nightmare Purple is noticeably darker, 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.
See Nightmare Purple on the colour wheel See Deep Magenta on the colour wheel
All matches: Nightmare Purple equivalents · Deep Magenta equivalents