Sapphire Gem vs Ultramarine Highlight

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Ultramarine Highlight is more muted than Sapphire Gem.

Can I use Ultramarine Highlight instead of Sapphire Gem?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Ultramarine Highlight is more muted than Sapphire Gem.

Can I use Sapphire Gem instead of Ultramarine Highlight?

The same difference applies the other way round: Sapphire Gem is 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.