ΔE 16.1, loose: different enough that you should pick on purpose. Green Sky is noticeably lighter than Chromium Oxide Green Dark.
Can I use Green Sky instead of Chromium Oxide Green Dark?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Green Sky is noticeably lighter than Chromium Oxide Green Dark.
Can I use Chromium Oxide Green Dark instead of Green Sky?
The same difference applies the other way round: Chromium Oxide Green Dark is noticeably darker.
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.