ΔE 23.4, loose: different enough that you should pick on purpose. Nightmare Purple is noticeably darker, much more muted than Magecast Magenta.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 23). Nightmare Purple is noticeably darker, much more muted than Magecast Magenta.
The same difference applies the other way round: Magecast 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.
See Magecast Magenta on the colour wheel See Nightmare Purple on the colour wheel
All matches: Magecast Magenta equivalents · Nightmare Purple equivalents