Leviathan Blue vs Heretic Turquoise

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Heretic Turquoise is a little darker, more muted, warmer than Leviathan Blue.

Can I use Heretic Turquoise instead of Leviathan Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Heretic Turquoise is a little darker, more muted, warmer than Leviathan Blue.

Can I use Leviathan Blue instead of Heretic Turquoise?

The same difference applies the other way round: Leviathan Blue is a little lighter, more saturated, 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.