Akhelian Green vs Dark Turquoise

How close are they?

ΔE 20.7, loose: different enough that you should pick on purpose. Dark Turquoise is a little darker, more muted, cooler than Akhelian Green.

Can I use Dark Turquoise instead of Akhelian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dark Turquoise is a little darker, more muted, cooler than Akhelian Green.

Can I use Akhelian Green instead of Dark Turquoise?

The same difference applies the other way round: Akhelian Green is a little lighter, more saturated, warmer.

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.