Grey Teal vs Green Sky

How close are they?

ΔE 22.3, loose: different enough that you should pick on purpose. Green Sky is much more saturated, warmer than Grey Teal.

Can I use Green Sky instead of Grey Teal?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Green Sky is much more saturated, warmer than Grey Teal.

Can I use Grey Teal instead of Green Sky?

The same difference applies the other way round: Grey Teal is much more muted, 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.