ΔE 38.2, loose: different enough that you should pick on purpose. Phantom Violet is noticeably darker, much more muted, cooler than Warlock Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Phantom Violet is noticeably darker, much more muted, cooler than Warlock Purple.
The same difference applies the other way round: Warlock Purple is noticeably lighter, much more saturated, 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 Warlock Purple on the colour wheel See Phantom Violet on the colour wheel
All matches: Warlock Purple equivalents · Phantom Violet equivalents