Triumphant Navy vs Prussian Blue

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Prussian Blue is more saturated than Triumphant Navy.

Can I use Prussian Blue instead of Triumphant Navy?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Prussian Blue is more saturated than Triumphant Navy.

Can I use Triumphant Navy instead of Prussian Blue?

The same difference applies the other way round: Triumphant Navy is 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.