Phthalo Turquoise Light vs Light Turquoise

How close are they?

ΔE 16.8, loose: different enough that you should pick on purpose. Light Turquoise is noticeably darker, more saturated than Phthalo Turquoise Light.

Can I use Light Turquoise instead of Phthalo Turquoise Light?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Light Turquoise is noticeably darker, more saturated than Phthalo Turquoise Light.

Can I use Phthalo Turquoise Light instead of Light Turquoise?

The same difference applies the other way round: Phthalo Turquoise Light is noticeably lighter, more muted.

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.