Akhelian Green vs Dark Green

How close are they?

ΔE 23.5, loose: different enough that you should pick on purpose. Dark Green is noticeably darker, warmer than Akhelian Green.

Can I use Dark Green instead of Akhelian Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Dark Green is noticeably darker, warmer than Akhelian Green.

Can I use Akhelian Green instead of Dark Green?

The same difference applies the other way round: Akhelian Green is noticeably lighter, 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.