Dark Navy Blue vs Violet

How close are they?

ΔE 8.3, close: similar, with a visible difference side by side. Violet is a little lighter, warmer than Dark Navy Blue.

Can I use Violet instead of Dark Navy Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Violet is a little lighter, warmer than Dark Navy Blue.

Can I use Dark Navy Blue instead of Violet?

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