ΔE 9.8, close: similar, with a visible difference side by side. Ash Grey is a little lighter, more muted than Putrid Flesh.
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Ash Grey is a little lighter, more muted than Putrid Flesh.
The same difference applies the other way round: Putrid Flesh is a little darker, 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 Putrid Flesh on the colour wheel See Ash Grey on the colour wheel
All matches: Putrid Flesh equivalents · Ash Grey equivalents