Imperial Navy vs Ultramarine Blue

How close are they?

ΔE 8.4, close: similar, with a visible difference side by side. Ultramarine Blue is a little lighter, cooler than Imperial Navy.

Can I use Ultramarine Blue instead of Imperial Navy?

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

Can I use Imperial Navy instead of Ultramarine Blue?

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

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.