Deep Blue vs Ritterlich Blue

How close are they?

ΔE 9.9, close: similar, with a visible difference side by side. Ritterlich Blue is more muted, cooler than Deep Blue.

Can I use Ritterlich Blue instead of Deep Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Ritterlich Blue is more muted, cooler than Deep Blue.

Can I use Deep Blue instead of Ritterlich Blue?

The same difference applies the other way round: Deep Blue 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.