Turquoise Siren vs Morning Sky

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Morning Sky is noticeably darker, more muted than Turquoise Siren.

Can I use Morning Sky instead of Turquoise Siren?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Morning Sky is noticeably darker, more muted than Turquoise Siren.

Can I use Turquoise Siren instead of Morning Sky?

The same difference applies the other way round: Turquoise Siren 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.