ΔE 19.4, loose: different enough that you should pick on purpose. Flanker Light Blue is noticeably darker, more muted than Light Phthalo Green.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Flanker Light Blue is noticeably darker, more muted than Light Phthalo Green.
The same difference applies the other way round: Light Phthalo Green is noticeably lighter, more saturated.
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 Flanker Light Blue on the colour wheel
All matches: Light Phthalo Green equivalents · Flanker Light Blue equivalents