Putrid Green vs Green Sky

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Green Sky is a little lighter, more muted, cooler than Putrid Green.

Can I use Green Sky instead of Putrid Green?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Green Sky is a little lighter, more muted, cooler than Putrid Green.

Can I use Putrid Green instead of Green Sky?

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