ΔE 17.1, loose: different enough that you should pick on purpose. Imperial Purple is noticeably darker, more saturated, warmer than Warpfiend Grey.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Imperial Purple is noticeably darker, more saturated, warmer than Warpfiend Grey.
The same difference applies the other way round: Warpfiend Grey is noticeably lighter, more muted, cooler.
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 Warpfiend Grey on the colour wheel See Imperial Purple on the colour wheel
All matches: Warpfiend Grey equivalents · Imperial Purple equivalents