Majestic Fortress vs Surfer Orc Flesh

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Surfer Orc Flesh is noticeably lighter, more saturated than Majestic Fortress.

Can I use Surfer Orc Flesh instead of Majestic Fortress?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Surfer Orc Flesh is noticeably lighter, more saturated than Majestic Fortress.

Can I use Majestic Fortress instead of Surfer Orc Flesh?

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

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.