ΔE 12.7, fair: the same colour family, but not a straight swap. Nacar is noticeably lighter than Longbeard Grey.
Can I use Nacar instead of Longbeard Grey?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Nacar is noticeably lighter than Longbeard Grey.
Can I use Longbeard Grey instead of Nacar?
The same difference applies the other way round: Longbeard Grey is noticeably darker.
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.