Deep Azure vs Phthalo Turquoise Light

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Phthalo Turquoise Light is noticeably lighter, cooler than Deep Azure.

Can I use Phthalo Turquoise Light instead of Deep Azure?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Phthalo Turquoise Light is noticeably lighter, cooler than Deep Azure.

Can I use Deep Azure instead of Phthalo Turquoise Light?

The same difference applies the other way round: Deep Azure is noticeably darker, 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.