Tyrian Navy vs Heretic Turquoise

How close are they?

ΔE 15.5, loose: different enough that you should pick on purpose. Heretic Turquoise is a little lighter, more saturated, warmer than Tyrian Navy.

Can I use Heretic Turquoise instead of Tyrian Navy?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Heretic Turquoise is a little lighter, more saturated, warmer than Tyrian Navy.

Can I use Tyrian Navy instead of Heretic Turquoise?

The same difference applies the other way round: Tyrian Navy is a little darker, 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.