Gothic Blue vs Cerulean Blue Deep

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Cerulean Blue Deep is cooler than Gothic Blue.

Can I use Cerulean Blue Deep instead of Gothic Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Cerulean Blue Deep is cooler than Gothic Blue.

Can I use Gothic Blue instead of Cerulean Blue Deep?

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