Green Hell vs Vulkan Green

How close are they?

ΔE 9.4, close: similar, with a visible difference side by side. Vulkan Green is a little darker, warmer than Green Hell.

Can I use Vulkan Green instead of Green Hell?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Vulkan Green is a little darker, warmer than Green Hell.

Can I use Green Hell instead of Vulkan Green?

The same difference applies the other way round: Green Hell is a little lighter, 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.