Tidal Blue vs Phthalo Turquoise Light

How close are they?

ΔE 17.4, loose: different enough that you should pick on purpose. Phthalo Turquoise Light is noticeably lighter, more muted than Tidal Blue.

Can I use Phthalo Turquoise Light instead of Tidal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Phthalo Turquoise Light is noticeably lighter, more muted than Tidal Blue.

Can I use Tidal Blue instead of Phthalo Turquoise Light?

The same difference applies the other way round: Tidal Blue is noticeably darker, 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.