ΔE 18.6, loose: different enough that you should pick on purpose. Slaanesh Grey is 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 Slaanesh 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 19). Slaanesh Grey is much more muted, cooler than Violet. They are different kinds of paint.
The same difference applies the other way round: Violet is 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 Slaanesh Grey on the colour wheel
All matches: Violet equivalents · Slaanesh Grey equivalents