ΔE 25.8, loose: different enough that you should pick on purpose. Light Phthalo Blue is noticeably lighter, much more muted, cooler than Phantasmal Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Light Phthalo Blue is noticeably lighter, much more muted, cooler than Phantasmal Blue.
The same difference applies the other way round: Phantasmal Blue is noticeably darker, much more saturated, warmer.
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 Phantasmal Blue on the colour wheel See Light Phthalo Blue on the colour wheel
All matches: Phantasmal Blue equivalents · Light Phthalo Blue equivalents