Nurgling Green vs Undead Flesh

How close are they?

ΔE 18.0, loose: different enough that you should pick on purpose. Undead Flesh is noticeably lighter, more muted, warmer than Nurgling Green.

Can I use Undead Flesh instead of Nurgling Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Undead Flesh is noticeably lighter, more muted, warmer than Nurgling Green.

Can I use Nurgling Green instead of Undead Flesh?

The same difference applies the other way round: Nurgling Green is noticeably darker, more saturated, 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.