Bright Sapphire vs Ion Blue

How close are they?

ΔE 17.9, loose: different enough that you should pick on purpose. Ion Blue is a little lighter, much more muted, warmer than Bright Sapphire.

Can I use Ion Blue instead of Bright Sapphire?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ion Blue is a little lighter, much more muted, warmer than Bright Sapphire.

Can I use Bright Sapphire instead of Ion Blue?

The same difference applies the other way round: Bright Sapphire is a little darker, much 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.