Triumphant Navy vs Ultramar Blue

How close are they?

ΔE 8.7, close: similar, with a visible difference side by side. Ultramar Blue is a little darker than Triumphant Navy.

Can I use Ultramar Blue instead of Triumphant Navy?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Ultramar Blue is a little darker than Triumphant Navy.

Can I use Triumphant Navy instead of Ultramar Blue?

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

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.