Ritterlich Blue vs Deep Sky

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Deep Sky is warmer than Ritterlich Blue.

Can I use Deep Sky instead of Ritterlich Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Deep Sky is warmer than Ritterlich Blue.

Can I use Ritterlich Blue instead of Deep Sky?

The same difference applies the other way round: Ritterlich Blue is 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.