ΔE 9.6, close: similar, with a visible difference side by side. Terminatus Stone is more saturated than Great Hall Grey.
Can I use Terminatus Stone instead of Great Hall Grey?
Usually, if you are not matching paint already on the model. Close match (ΔE 10). Terminatus Stone is more saturated than Great Hall Grey.
Can I use Great Hall Grey instead of Terminatus Stone?
The same difference applies the other way round: Great Hall Grey is more muted.
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.