Sapphire Gem vs Kingdom Blue

How close are they?

ΔE 19.4, loose: different enough that you should pick on purpose. Kingdom Blue is a little darker, more saturated, warmer than Sapphire Gem.

Can I use Kingdom Blue instead of Sapphire Gem?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Kingdom Blue is a little darker, more saturated, warmer than Sapphire Gem.

Can I use Sapphire Gem instead of Kingdom Blue?

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