ΔE 21.0, loose: different enough that you should pick on purpose. Frostveil is a little lighter, much more saturated, cooler than Necromancer Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Frostveil is a little lighter, much more saturated, cooler than Necromancer Purple.
The same difference applies the other way round: Necromancer Purple is a little darker, much more muted, warmer.
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 Frostveil on the colour wheel
All matches: Necromancer Purple equivalents · Frostveil equivalents