ΔE 22.4, loose: different enough that you should pick on purpose. USN Light Ghost Grey is noticeably darker than Underbelly Blue.
Can I use USN Light Ghost Grey instead of Underbelly Blue?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). USN Light Ghost Grey is noticeably darker than Underbelly Blue.
Can I use Underbelly Blue instead of USN Light Ghost Grey?
The same difference applies the other way round: Underbelly Blue 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.