ΔE 19.1, loose: different enough that you should pick on purpose. Dark Green is noticeably darker, more muted, warmer than Phthalo Green (Blue Shade).
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Dark Green is noticeably darker, more muted, warmer than Phthalo Green (Blue Shade).
The same difference applies the other way round: Phthalo Green (Blue Shade) is noticeably lighter, more saturated, 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 (Blue Shade) on the colour wheel See Dark Green on the colour wheel
All matches: Phthalo Green (Blue Shade) equivalents · Dark Green equivalents