Crystal Blue vs Ultramarine Highlight

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Ultramarine Highlight is a little darker, more muted than Crystal Blue.

Can I use Ultramarine Highlight instead of Crystal Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ultramarine Highlight is a little darker, more muted than Crystal Blue.

Can I use Crystal Blue instead of Ultramarine Highlight?

The same difference applies the other way round: Crystal Blue is a little lighter, more saturated.

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.