Turquoise Siren vs Spirit Blue

How close are they?

ΔE 8.3, close: similar, with a visible difference side by side. Spirit Blue is more muted, cooler than Turquoise Siren.

Can I use Spirit Blue instead of Turquoise Siren?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Spirit Blue is more muted, cooler than Turquoise Siren.

Can I use Turquoise Siren instead of Spirit Blue?

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