Purple Flesh vs Slaanesh Grey

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Slaanesh Grey is a little lighter, much more muted, cooler than Purple Flesh.

Can I use Slaanesh Grey instead of Purple Flesh?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Slaanesh Grey is a little lighter, much more muted, cooler than Purple Flesh.

Can I use Purple Flesh instead of Slaanesh Grey?

The same difference applies the other way round: Purple Flesh is a little darker, 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.