Hydra Turquoise vs Turquois (Phthalo)

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Turquois (Phthalo) is noticeably darker, more muted than Hydra Turquoise.

Can I use Turquois (Phthalo) instead of Hydra Turquoise?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Turquois (Phthalo) is noticeably darker, more muted than Hydra Turquoise.

Can I use Hydra Turquoise instead of Turquois (Phthalo)?

The same difference applies the other way round: Hydra Turquoise is noticeably lighter, more saturated.

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.