Seawater vs Majestic Fortress

How close are they?

ΔE 6.4, close: similar, with a visible difference side by side. Majestic Fortress is a little lighter than Seawater.

Can I use Majestic Fortress instead of Seawater?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Majestic Fortress is a little lighter than Seawater.

Can I use Seawater instead of Majestic Fortress?

The same difference applies the other way round: Seawater is a little darker.

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.