Twilight Sky vs Turquoise

How close are they?

ΔE 9.6, close: similar, with a visible difference side by side. Turquoise is a little darker, more muted than Twilight Sky.

Can I use Turquoise instead of Twilight Sky?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Turquoise is a little darker, more muted than Twilight Sky.

Can I use Twilight Sky instead of Turquoise?

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