Majestic Fortress vs Arphen Jade

How close are they?

ΔE 17.2, loose: different enough that you should pick on purpose. Arphen Jade is a little darker, much more muted, cooler than Majestic Fortress.

Can I use Arphen Jade instead of Majestic Fortress?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Arphen Jade is a little darker, much more muted, cooler than Majestic Fortress.

Can I use Majestic Fortress instead of Arphen Jade?

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

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.