Forest of Shadows vs Green Sky

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Green Sky is a little lighter, warmer than Forest of Shadows.

Can I use Green Sky instead of Forest of Shadows?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Green Sky is a little lighter, warmer than Forest of Shadows.

Can I use Forest of Shadows instead of Green Sky?

The same difference applies the other way round: Forest of Shadows is a little darker, 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.