ΔE 22.1, loose: different enough that you should pick on purpose. Slaanesh Grey is noticeably darker, much more muted than Lavender.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Slaanesh Grey is noticeably darker, much more muted than Lavender.
The same difference applies the other way round: Lavender is noticeably 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 Lavender on the colour wheel See Slaanesh Grey on the colour wheel
All matches: Lavender equivalents · Slaanesh Grey equivalents