Gothic Blue vs Sapphire Blue

How close are they?

ΔE 8.3, close: similar, with a visible difference side by side. Sapphire Blue is more saturated, cooler than Gothic Blue.

Can I use Sapphire Blue instead of Gothic Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Sapphire Blue is more saturated, cooler than Gothic Blue.

Can I use Gothic Blue instead of Sapphire Blue?

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