Necromancer Cloak vs Blue Mountain

How close are they?

ΔE 6.9, close: similar, with a visible difference side by side. Blue Mountain is more saturated than Necromancer Cloak.

Can I use Blue Mountain instead of Necromancer Cloak?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Blue Mountain is more saturated than Necromancer Cloak.

Can I use Necromancer Cloak instead of Blue Mountain?

The same difference applies the other way round: Necromancer Cloak is 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.