ΔE 16.7, loose: different enough that you should pick on purpose. Warpfiend Grey is noticeably lighter, cooler than Demon Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Warpfiend Grey is noticeably lighter, cooler than Demon Purple.
The same difference applies the other way round: Demon Purple is noticeably darker, 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 Demon Purple on the colour wheel See Warpfiend Grey on the colour wheel
All matches: Demon Purple equivalents · Warpfiend Grey equivalents