ΔE 6.6, close: similar, with a visible difference side by side. Longbeard Grey is a little darker than Great Hall Grey.
Can I use Longbeard Grey instead of Great Hall Grey?
Usually, if you are not matching paint already on the model. Close match (ΔE 7). Longbeard Grey is a little darker than Great Hall Grey.
Can I use Great Hall Grey instead of Longbeard Grey?
The same difference applies the other way round: Great Hall Grey is a little lighter.
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.