Sapphire Gem vs Cygnar Blue Base

How close are they?

ΔE 16.2, loose: different enough that you should pick on purpose. Cygnar Blue Base is noticeably darker, more saturated, warmer than Sapphire Gem.

Can I use Cygnar Blue Base instead of Sapphire Gem?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Cygnar Blue Base is noticeably darker, more saturated, warmer than Sapphire Gem.

Can I use Sapphire Gem instead of Cygnar Blue Base?

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