ΔE 26.5, loose: different enough that you should pick on purpose. Green Sky is noticeably lighter, more saturated, warmer than Forest Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Green Sky is noticeably lighter, more saturated, warmer than Forest Green.
The same difference applies the other way round: Forest Green is noticeably darker, 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.
See Forest Green on the colour wheel See Green Sky on the colour wheel
All matches: Forest Green equivalents · Green Sky equivalents