Aquamarine vs Light Phthalo Blue

How close are they?

ΔE 20.2, loose: different enough that you should pick on purpose. Light Phthalo Blue is noticeably lighter, more muted, cooler than Aquamarine.

Can I use Light Phthalo Blue instead of Aquamarine?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Light Phthalo Blue is noticeably lighter, more muted, cooler than Aquamarine.

Can I use Aquamarine instead of Light Phthalo Blue?

The same difference applies the other way round: Aquamarine is noticeably darker, 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.