Sapphire Gem vs Relic Blue

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Relic Blue is a little lighter, more muted, cooler than Sapphire Gem.

Can I use Relic Blue instead of Sapphire Gem?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Relic Blue is a little lighter, more muted, cooler than Sapphire Gem.

Can I use Sapphire Gem instead of Relic Blue?

The same difference applies the other way round: Sapphire Gem is a little darker, 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.