Majestic Fortress vs Phthalo Green (Blue Shade)

How close are they?

ΔE 9.9, close: similar, with a visible difference side by side. Phthalo Green (Blue Shade) is more saturated, warmer than Majestic Fortress.

Can I use Phthalo Green (Blue Shade) instead of Majestic Fortress?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Phthalo Green (Blue Shade) is more saturated, warmer than Majestic Fortress.

Can I use Majestic Fortress instead of Phthalo Green (Blue Shade)?

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.