ΔE 20.8, loose: different enough that you should pick on purpose. Light Green (Yellow Shade) is noticeably lighter than Leafy Green.
Can I use Light Green (Yellow Shade) instead of Leafy Green?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Light Green (Yellow Shade) is noticeably lighter than Leafy Green.
Can I use Leafy Green instead of Light Green (Yellow Shade)?
The same difference applies the other way round: Leafy Green 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.