Turquoise Ghost vs Heretic Turquoise

How close are they?

ΔE 9.8, close: similar, with a visible difference side by side. Heretic Turquoise is more muted, cooler than Turquoise Ghost.

Can I use Heretic Turquoise instead of Turquoise Ghost?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Heretic Turquoise is more muted, cooler than Turquoise Ghost.

Can I use Turquoise Ghost instead of Heretic Turquoise?

The same difference applies the other way round: Turquoise Ghost is more saturated, warmer.

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.