Necrotic Flesh vs Undead Flesh

How close are they?

ΔE 7.5, close: similar, with a visible difference side by side. Undead Flesh is warmer than Necrotic Flesh.

Can I use Undead Flesh instead of Necrotic Flesh?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Undead Flesh is warmer than Necrotic Flesh.

Can I use Necrotic Flesh instead of Undead Flesh?

The same difference applies the other way round: Necrotic Flesh is cooler.

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.