ΔE 15.9, loose: different enough that you should pick on purpose. Warpfiend Grey is a little darker, much more muted than Hexed Violet.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Warpfiend Grey is a little darker, much more muted than Hexed Violet.
The same difference applies the other way round: Hexed Violet is a little lighter, much more saturated.
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 Hexed Violet on the colour wheel See Warpfiend Grey on the colour wheel
All matches: Hexed Violet equivalents · Warpfiend Grey equivalents