Turquoise Interference vs Sapphire Blue

How close are they?

ΔE 17.6, loose: different enough that you should pick on purpose. Sapphire Blue is a little lighter, more saturated than Turquoise Interference.

Can I use Sapphire Blue instead of Turquoise Interference?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Sapphire Blue is a little lighter, more saturated than Turquoise Interference.

Can I use Turquoise Interference instead of Sapphire Blue?

The same difference applies the other way round: Turquoise Interference is a little darker, 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.