ΔE 63.8, loose: different enough that you should pick on purpose. Ink Green is noticeably darker, much more saturated, warmer than Light Phthalo Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 64). Ink Green is noticeably darker, much more saturated, warmer than Light Phthalo Green.
The same difference applies the other way round: Light Phthalo Green is noticeably lighter, 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.
See Light Phthalo Green on the colour wheel See Ink Green on the colour wheel
All matches: Light Phthalo Green equivalents · Ink Green equivalents