Cold Green vs Majestic Fortress

How close are they?

ΔE 7.4, close: similar, with a visible difference side by side. Majestic Fortress is a little darker, warmer than Cold Green.

Can I use Majestic Fortress instead of Cold Green?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Majestic Fortress is a little darker, warmer than Cold Green.

Can I use Cold Green instead of Majestic Fortress?

The same difference applies the other way round: Cold Green 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.