ΔE 17.1, loose: different enough that you should pick on purpose. Deep Sky Blue is a little lighter, more saturated, warmer than Phthalo Turquoise Light.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Deep Sky Blue is a little lighter, more saturated, warmer than Phthalo Turquoise Light.
The same difference applies the other way round: Phthalo Turquoise Light is a little darker, more muted, 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.
See Phthalo Turquoise Light on the colour wheel See Deep Sky Blue on the colour wheel
All matches: Phthalo Turquoise Light equivalents · Deep Sky Blue equivalents