Bright Sapphire vs Decadent Purple

How close are they?

ΔE 19.7, loose: different enough that you should pick on purpose. Decadent Purple is more muted, warmer than Bright Sapphire.

Can I use Decadent Purple instead of Bright Sapphire?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Decadent Purple is more muted, warmer than Bright Sapphire.

Can I use Bright Sapphire instead of Decadent Purple?

The same difference applies the other way round: Bright Sapphire is more saturated, 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.