Twilight Sky vs Prussian Blue

How close are they?

ΔE 7.4, close: similar, with a visible difference side by side. Prussian Blue is a little darker, warmer than Twilight Sky.

Can I use Prussian Blue instead of Twilight Sky?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Prussian Blue is a little darker, warmer than Twilight Sky.

Can I use Twilight Sky instead of Prussian Blue?

The same difference applies the other way round: Twilight Sky is a little lighter, cooler.

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.