Twilight Sky vs Turquois (Phthalo)

How close are they?

ΔE 20.1, loose: different enough that you should pick on purpose. Turquois (Phthalo) is a little lighter, more muted than Twilight Sky.

Can I use Turquois (Phthalo) instead of Twilight Sky?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Turquois (Phthalo) is a little lighter, more muted than Twilight Sky.

Can I use Twilight Sky instead of Turquois (Phthalo)?

The same difference applies the other way round: Twilight Sky is a little 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.