Majestic Fortress vs Greenskin Flesh

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Greenskin Flesh is more saturated, warmer than Majestic Fortress.

Can I use Greenskin Flesh instead of Majestic Fortress?

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

Can I use Majestic Fortress instead of Greenskin Flesh?

The same difference applies the other way round: Majestic Fortress is 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.