Sapphire Gem vs Exile Blue

How close are they?

ΔE 27.4, loose: different enough that you should pick on purpose. Exile Blue is noticeably darker, much more muted, warmer than Sapphire Gem.

Can I use Exile Blue instead of Sapphire Gem?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Exile Blue is noticeably darker, much more muted, warmer than Sapphire Gem.

Can I use Sapphire Gem instead of Exile Blue?

The same difference applies the other way round: Sapphire Gem is noticeably lighter, much 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.