Turquoise Siren vs Light Turquois (Phthalo)

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Light Turquois (Phthalo) is noticeably darker, warmer than Turquoise Siren.

Can I use Light Turquois (Phthalo) instead of Turquoise Siren?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Light Turquois (Phthalo) is noticeably darker, warmer than Turquoise Siren.

Can I use Turquoise Siren instead of Light Turquois (Phthalo)?

The same difference applies the other way round: Turquoise Siren 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.