Sapphire Blue vs Smoke Ink

How close are they?

ΔE 38.3, loose: different enough that you should pick on purpose. Smoke Ink is noticeably darker, much more muted than Sapphire Blue.

Can I use Smoke Ink instead of Sapphire Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 38). Smoke Ink is noticeably darker, much more muted than Sapphire Blue.

Can I use Sapphire Blue instead of Smoke Ink?

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

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.