Orc Green vs Green Sky

How close are they?

ΔE 20.6, loose: different enough that you should pick on purpose. Green Sky is a little lighter, much more muted than Orc Green.

Can I use Green Sky instead of Orc Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Green Sky is a little lighter, much more muted than Orc Green.

Can I use Orc Green instead of Green Sky?

The same difference applies the other way round: Orc Green is a little darker, much more saturated.

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.