ΔE 16.6, loose: different enough that you should pick on purpose. Darkness Grey is noticeably lighter, more muted than Navy Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Darkness Grey is noticeably lighter, more muted than Navy Blue.
The same difference applies the other way round: Navy Blue is noticeably darker, 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.
See Navy Blue on the colour wheel See Darkness Grey on the colour wheel
All matches: Navy Blue equivalents · Darkness Grey equivalents