Darth Blue vs Sapphire Blue

How close are they?

ΔE 25.8, loose: different enough that you should pick on purpose. Sapphire Blue is noticeably lighter, more muted, cooler than Darth Blue.

Can I use Sapphire Blue instead of Darth Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Sapphire Blue is noticeably lighter, more muted, cooler than Darth Blue.

Can I use Darth Blue instead of Sapphire Blue?

The same difference applies the other way round: Darth Blue is noticeably 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.