ΔE 15.2, loose: different enough that you should pick on purpose. Light Turquoise is a little lighter, more saturated, cooler than Turquois (Phthalo).
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Light Turquoise is a little lighter, more saturated, cooler than Turquois (Phthalo).
The same difference applies the other way round: Turquois (Phthalo) is a little darker, more muted, 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.
See Turquois (Phthalo) on the colour wheel See Light Turquoise on the colour wheel
All matches: Turquois (Phthalo) equivalents · Light Turquoise equivalents