Green Hell vs Nurgling Green

How close are they?

ΔE 25.4, loose: different enough that you should pick on purpose. Nurgling Green is noticeably lighter, warmer than Green Hell.

Can I use Nurgling Green instead of Green Hell?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Nurgling Green is noticeably lighter, warmer than Green Hell.

Can I use Green Hell instead of Nurgling Green?

The same difference applies the other way round: Green Hell is noticeably darker, 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.