Dark Navy Blue vs Purple

How close are they?

ΔE 16.6, loose: different enough that you should pick on purpose. Purple is noticeably lighter, more saturated, warmer than Dark Navy Blue.

Can I use Purple instead of Dark Navy Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Purple is noticeably lighter, more saturated, warmer than Dark Navy Blue.

Can I use Dark Navy Blue instead of Purple?

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

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.