Turquoise Ink vs Ink Dark Turquoise

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Ink Dark Turquoise is a little lighter, warmer than Turquoise Ink.

Can I use Ink Dark Turquoise instead of Turquoise Ink?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Ink Dark Turquoise is a little lighter, warmer than Turquoise Ink.

Can I use Turquoise Ink instead of Ink Dark Turquoise?

The same difference applies the other way round: Turquoise Ink is a little darker, 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.