ΔE 16.0, loose: different enough that you should pick on purpose. Slaanesh Grey is a little darker, much more muted, cooler than Matt Violet. They are different kinds of paint: AMMO by Mig Matt 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 16). Slaanesh Grey is a little darker, much more muted, cooler than Matt Violet. They are different kinds of paint.
The same difference applies the other way round: Matt 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 Matt Violet on the colour wheel See Slaanesh Grey on the colour wheel
All matches: Matt Violet equivalents · Slaanesh Grey equivalents