Algae Green vs Dark Green

How close are they?

ΔE 45.1, loose: different enough that you should pick on purpose. Dark Green is noticeably darker, much more muted, cooler than Algae Green.

Can I use Dark Green instead of Algae Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 45). Dark Green is noticeably darker, much more muted, cooler than Algae Green.

Can I use Algae Green instead of Dark Green?

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