ΔE 10.8, fair: the same colour family, but not a straight swap. Dark Ivory is noticeably lighter than Longbeard Grey.
Can I use Dark Ivory instead of Longbeard Grey?
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Dark Ivory is noticeably lighter than Longbeard Grey.
Can I use Longbeard Grey instead of Dark Ivory?
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.