ΔE 20.4, loose: different enough that you should pick on purpose. Primer Dark Neutral Grey is noticeably darker than Astrogranite.
Can I use Primer Dark Neutral Grey instead of Astrogranite?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Primer Dark Neutral Grey is noticeably darker than Astrogranite.
Can I use Astrogranite instead of Primer Dark Neutral Grey?
The same difference applies the other way round: Astrogranite is noticeably 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.