Slaanesh Grey vs Light Violet

How close are they?

ΔE 18.6, loose: different enough that you should pick on purpose. Light Violet is a little lighter, much more saturated than Slaanesh Grey.

Can I use Light Violet instead of Slaanesh Grey?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Light Violet is a little lighter, much more saturated than Slaanesh Grey.

Can I use Slaanesh Grey instead of Light Violet?

The same difference applies the other way round: Slaanesh Grey is a little darker, much more muted.

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.