Matt Ash Black vs Ink Dark Turquoise

How close are they?

ΔE 18.6, loose: different enough that you should pick on purpose. Ink Dark Turquoise is noticeably darker, more saturated than Matt Ash Black.

Can I use Ink Dark Turquoise instead of Matt Ash Black?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Ink Dark Turquoise is noticeably darker, more saturated than Matt Ash Black.

Can I use Matt Ash Black instead of Ink Dark Turquoise?

The same difference applies the other way round: Matt Ash Black is noticeably lighter, 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.