Sapphire Gem vs Cyan Blue

How close are they?

ΔE 8.8, close: similar, with a visible difference side by side. Cyan Blue is more muted, cooler than Sapphire Gem.

Can I use Cyan Blue instead of Sapphire Gem?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Cyan Blue is more muted, cooler than Sapphire Gem.

Can I use Sapphire Gem instead of Cyan Blue?

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