ΔE 26.3, loose: different enough that you should pick on purpose. Magecast Magenta is much more saturated, warmer than Necromancer Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Magecast Magenta is much more saturated, warmer than Necromancer Purple.
The same difference applies the other way round: Necromancer Purple is much 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.
See Necromancer Purple on the colour wheel See Magecast Magenta on the colour wheel
All matches: Necromancer Purple equivalents · Magecast Magenta equivalents