Turquoise Siren vs Spectral Blue

How close are they?

ΔE 22.2, loose: different enough that you should pick on purpose. Spectral Blue is noticeably darker, more muted, cooler than Turquoise Siren.

Can I use Spectral Blue instead of Turquoise Siren?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Spectral Blue is noticeably darker, more muted, cooler than Turquoise Siren.

Can I use Turquoise Siren instead of Spectral Blue?

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