Majestic Fortress vs Green Hell

How close are they?

ΔE 6.5, close: similar, with a visible difference side by side. Green Hell is a little darker, more muted than Majestic Fortress.

Can I use Green Hell instead of Majestic Fortress?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Green Hell is a little darker, more muted than Majestic Fortress.

Can I use Majestic Fortress instead of Green Hell?

The same difference applies the other way round: Majestic Fortress is a little lighter, more saturated.

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.