Putrid Flesh vs Slaanesh Grey

How close are they?

ΔE 9.6, close: similar, with a visible difference side by side. Slaanesh Grey is more saturated, cooler than Putrid Flesh.

Can I use Slaanesh Grey instead of Putrid Flesh?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Slaanesh Grey is more saturated, cooler than Putrid Flesh.

Can I use Putrid Flesh instead of Slaanesh Grey?

The same difference applies the other way round: Putrid Flesh is more muted, 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.