Triumphant Navy vs Greatcoat Grey

How close are they?

ΔE 24.8, loose: different enough that you should pick on purpose. Greatcoat Grey is noticeably lighter, much more muted, cooler than Triumphant Navy.

Can I use Greatcoat Grey instead of Triumphant Navy?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Greatcoat Grey is noticeably lighter, much more muted, cooler than Triumphant Navy.

Can I use Triumphant Navy instead of Greatcoat Grey?

The same difference applies the other way round: Triumphant Navy is noticeably darker, much more saturated, 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.