ΔE 21.6, loose: different enough that you should pick on purpose. Splinter Green is noticeably darker, warmer than Phthalo Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Splinter Green is noticeably darker, warmer than Phthalo Green.
The same difference applies the other way round: Phthalo Green is noticeably lighter, 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 Phthalo Green on the colour wheel See Splinter Green on the colour wheel
All matches: Phthalo Green equivalents · Splinter Green equivalents