Neptunus Blue vs Sapphire Blue

How close are they?

ΔE 22.2, loose: different enough that you should pick on purpose. Sapphire Blue is noticeably darker, more saturated than Neptunus Blue.

Can I use Sapphire Blue instead of Neptunus Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Sapphire Blue is noticeably darker, more saturated than Neptunus Blue.

Can I use Neptunus Blue instead of Sapphire Blue?

The same difference applies the other way round: Neptunus Blue is noticeably lighter, more muted.

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.