ΔE 12.2, fair: the same colour family, but not a straight swap. Bluebird Grey is a little lighter, more saturated, warmer than Necromancer Cloak.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Bluebird Grey is a little lighter, more saturated, warmer than Necromancer Cloak.
The same difference applies the other way round: Necromancer Cloak is a little darker, 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.
See Necromancer Cloak on the colour wheel See Bluebird Grey on the colour wheel
All matches: Necromancer Cloak equivalents · Bluebird Grey equivalents