ΔE 15.9, loose: different enough that you should pick on purpose. Dead Flush is more saturated than Administratum Grey.
Can I use Dead Flush instead of Administratum Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Dead Flush is more saturated than Administratum Grey.
Can I use Administratum Grey instead of Dead Flush?
The same difference applies the other way round: Administratum 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.