Tyrian Navy vs Leviathan Blue

How close are they?

ΔE 19.7, loose: different enough that you should pick on purpose. Leviathan Blue is noticeably lighter, much more saturated than Tyrian Navy.

Can I use Leviathan Blue instead of Tyrian Navy?

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

Can I use Tyrian Navy instead of Leviathan Blue?

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