Turquoise Siren vs Surf Aqua

How close are they?

ΔE 7.1, close: similar, with a visible difference side by side. Surf Aqua is a little darker, more muted than Turquoise Siren.

Can I use Surf Aqua instead of Turquoise Siren?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Surf Aqua is a little darker, more muted than Turquoise Siren.

Can I use Turquoise Siren instead of Surf Aqua?

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