Navy Blue vs Ultramarine Shadow

How close are they?

ΔE 6.1, close: similar, with a visible difference side by side. Ultramarine Shadow is a little darker, more saturated than Navy Blue.

Can I use Ultramarine Shadow instead of Navy Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Ultramarine Shadow is a little darker, more saturated than Navy Blue.

Can I use Navy Blue instead of Ultramarine Shadow?

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