Celestial Azure vs Hydra Turquoise

How close are they?

ΔE 17.2, loose: different enough that you should pick on purpose. Hydra Turquoise is noticeably darker, cooler than Celestial Azure.

Can I use Hydra Turquoise instead of Celestial Azure?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Hydra Turquoise is noticeably darker, cooler than Celestial Azure.

Can I use Celestial Azure instead of Hydra Turquoise?

The same difference applies the other way round: Celestial Azure is noticeably lighter, 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.