ΔE 17.1, loose: different enough that you should pick on purpose. Warpfiend Grey is a little darker, much more muted, cooler than Violet. They are different kinds of paint: AMMO by Mig Violet is an ink, glaze or transparent paint and Citadel Warpfiend Grey is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Warpfiend Grey is a little darker, much more muted, cooler than Violet. They are different kinds of paint.
The same difference applies the other way round: Violet is a little 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 Violet on the colour wheel See Warpfiend Grey on the colour wheel
All matches: Violet equivalents · Warpfiend Grey equivalents