Feral Green vs Green Hell

How close are they?

ΔE 26.8, loose: different enough that you should pick on purpose. Green Hell is noticeably darker, much more muted, cooler than Feral Green.

Can I use Green Hell instead of Feral Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Green Hell is noticeably darker, much more muted, cooler than Feral Green.

Can I use Feral Green instead of Green Hell?

The same difference applies the other way round: Feral Green is noticeably lighter, much more saturated, 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.