Aquamarine vs Light Turquois (Phthalo)

How close are they?

ΔE 21.6, loose: different enough that you should pick on purpose. Light Turquois (Phthalo) is noticeably darker, warmer than Aquamarine.

Can I use Light Turquois (Phthalo) instead of Aquamarine?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Light Turquois (Phthalo) is noticeably darker, warmer than Aquamarine.

Can I use Aquamarine instead of Light Turquois (Phthalo)?

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