Inktense Black vs Ink Dark Turquoise

How close are they?

ΔE 24.2, loose: different enough that you should pick on purpose. Ink Dark Turquoise is noticeably lighter, much more saturated than Inktense Black.

Can I use Ink Dark Turquoise instead of Inktense Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Ink Dark Turquoise is noticeably lighter, much more saturated than Inktense Black.

Can I use Inktense Black instead of Ink Dark Turquoise?

The same difference applies the other way round: Inktense Black is noticeably darker, much 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.