Triumphant Navy vs Blue Mountain

How close are they?

ΔE 20.3, loose: different enough that you should pick on purpose. Blue Mountain is noticeably lighter, much more muted than Triumphant Navy.

Can I use Blue Mountain instead of Triumphant Navy?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Blue Mountain is noticeably lighter, much more muted than Triumphant Navy.

Can I use Triumphant Navy instead of Blue Mountain?

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

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.