ΔE 16.7, loose: different enough that you should pick on purpose. USN Light Ghost Grey is a little lighter than Bright Brown Grey.
Can I use USN Light Ghost Grey instead of Bright Brown Grey?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). USN Light Ghost Grey is a little lighter than Bright Brown Grey.
Can I use Bright Brown Grey instead of USN Light Ghost Grey?
The same difference applies the other way round: Bright Brown Grey is a little darker.
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.