ΔE 13.8, fair: the same colour family, but not a straight swap. Imperial Blue is more saturated, cooler than Necromancer Purple.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Imperial Blue is more saturated, cooler than Necromancer Purple.
The same difference applies the other way round: Necromancer Purple is 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 Imperial Blue on the colour wheel
All matches: Necromancer Purple equivalents · Imperial Blue equivalents