ΔE 7.8, close: similar, with a visible difference side by side. Administratum Grey is a little darker than Grey Castle.
Can I use Administratum Grey instead of Grey Castle?
Usually, if you are not matching paint already on the model. Close match (ΔE 8). Administratum Grey is a little darker than Grey Castle.
Can I use Grey Castle instead of Administratum Grey?
The same difference applies the other way round: Grey Castle 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.