Ionic Blue vs Purple Rain

How close are they?

ΔE 29.8, loose: different enough that you should pick on purpose. Purple Rain is noticeably darker, more saturated, warmer than Ionic Blue.

Can I use Purple Rain instead of Ionic Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 30). Purple Rain is noticeably darker, more saturated, warmer than Ionic Blue.

Can I use Ionic Blue instead of Purple Rain?

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