ΔE 13.1, fair: the same colour family, but not a straight swap. Longbeard Grey is more muted than Undead Skin.
Can I use Longbeard Grey instead of Undead Skin?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Longbeard Grey is more muted than Undead Skin.
Can I use Undead Skin instead of Longbeard Grey?
The same difference applies the other way round: Undead Skin is more saturated.
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.