Dark Navy Blue vs Dark Blue

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Dark Blue is noticeably lighter, more saturated than Dark Navy Blue.

Can I use Dark Blue instead of Dark Navy Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Dark Blue is noticeably lighter, more saturated than Dark Navy Blue.

Can I use Dark Navy Blue instead of Dark Blue?

The same difference applies the other way round: Dark Navy Blue is noticeably darker, 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.