Crystal Cavern vs Ultramarine Blue

How close are they?

ΔE 6.3, close: similar, with a visible difference side by side. Ultramarine Blue is more muted, warmer than Crystal Cavern.

Can I use Ultramarine Blue instead of Crystal Cavern?

Usually, if you are not matching paint already on the model. Close match (ΔE 6). Ultramarine Blue is more muted, warmer than Crystal Cavern.

Can I use Crystal Cavern instead of Ultramarine Blue?

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