Green Sky vs Amulet Aqua

How close are they?

ΔE 17.3, loose: different enough that you should pick on purpose. Amulet Aqua is a little lighter, cooler than Green Sky.

Can I use Amulet Aqua instead of Green Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Amulet Aqua is a little lighter, cooler than Green Sky.

Can I use Green Sky instead of Amulet Aqua?

The same difference applies the other way round: Green Sky is a little darker, 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.