Triumphant Navy vs Dark Purple

How close are they?

ΔE 8.7, close: similar, with a visible difference side by side. Dark Purple is more saturated, warmer than Triumphant Navy.

Can I use Dark Purple instead of Triumphant Navy?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Dark Purple is more saturated, warmer than Triumphant Navy.

Can I use Triumphant Navy instead of Dark Purple?

The same difference applies the other way round: Triumphant Navy is more muted, cooler.

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.