Crystal Cavern vs Sapphire Blue

How close are they?

ΔE 9.5, close: similar, with a visible difference side by side. Sapphire Blue is more saturated, warmer than Crystal Cavern.

Can I use Sapphire Blue instead of Crystal Cavern?

Usually, if you are not matching paint already on the model. Close match (ΔE 10). Sapphire Blue is more saturated, warmer than Crystal Cavern.

Can I use Crystal Cavern instead of Sapphire Blue?

The same difference applies the other way round: Crystal Cavern is more muted, 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.