ΔE 19.2, loose: different enough that you should pick on purpose. Longbeard Grey is much more muted than Phantom Blue.
Can I use Longbeard Grey instead of Phantom Blue?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Longbeard Grey is much more muted than Phantom Blue.
Can I use Phantom Blue instead of Longbeard Grey?
The same difference applies the other way round: Phantom Blue is much more saturated.
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.