ΔE 18.2, loose: different enough that you should pick on purpose. Bright Green Blue is much more saturated than Longbeard Grey.
Can I use Bright Green Blue instead of Longbeard Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Bright Green Blue is much more saturated than Longbeard Grey.
Can I use Longbeard Grey instead of Bright Green Blue?
The same difference applies the other way round: Longbeard Grey is much 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.