Ashen Stone vs Dark Turquoise

How close are they?

ΔE 37.0, loose: different enough that you should pick on purpose. Dark Turquoise is noticeably darker, much more saturated than Ashen Stone.

Can I use Dark Turquoise instead of Ashen Stone?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 37). Dark Turquoise is noticeably darker, much more saturated than Ashen Stone.

Can I use Ashen Stone instead of Dark Turquoise?

The same difference applies the other way round: Ashen Stone is noticeably lighter, 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.