ΔE 25.2, loose: different enough that you should pick on purpose. Aquamarine is noticeably darker, much more saturated, warmer than Light Phthalo Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Aquamarine is noticeably darker, much more saturated, warmer than Light Phthalo Blue.
The same difference applies the other way round: Light Phthalo Blue 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 Blue on the colour wheel See Aquamarine on the colour wheel
All matches: Light Phthalo Blue equivalents · Aquamarine equivalents