Dark Grey vs Navy Blue

How close are they?

ΔE 8.7, close: similar, with a visible difference side by side. Navy Blue is a little lighter, more saturated than Dark Grey.

Can I use Navy Blue instead of Dark Grey?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Navy Blue is a little lighter, more saturated than Dark Grey.

Can I use Dark Grey instead of Navy Blue?

The same difference applies the other way round: Dark Grey is a little 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.