Majestic Fortress vs Leaf Green

How close are they?

ΔE 18.2, loose: different enough that you should pick on purpose. Leaf Green is much more saturated, warmer than Majestic Fortress.

Can I use Leaf Green instead of Majestic Fortress?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Leaf Green is much more saturated, warmer than Majestic Fortress.

Can I use Majestic Fortress instead of Leaf Green?

The same difference applies the other way round: Majestic Fortress is much more muted, 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.