Dark Green Blue vs Heretic Turquoise

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Heretic Turquoise is warmer than Dark Green Blue.

Can I use Heretic Turquoise instead of Dark Green Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Heretic Turquoise is warmer than Dark Green Blue.

Can I use Dark Green Blue instead of Heretic Turquoise?

The same difference applies the other way round: Dark Green Blue is 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.