Opulentus Black vs Ink Dark Turquoise

How close are they?

ΔE 18.7, loose: different enough that you should pick on purpose. Ink Dark Turquoise is a little lighter, more saturated than Opulentus Black.

Can I use Ink Dark Turquoise instead of Opulentus Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Ink Dark Turquoise is a little lighter, more saturated than Opulentus Black.

Can I use Opulentus Black instead of Ink Dark Turquoise?

The same difference applies the other way round: Opulentus Black is a little darker, 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.