Crystal Blue vs Purple Rain

How close are they?

ΔE 17.1, loose: different enough that you should pick on purpose. Purple Rain is a little darker, warmer than Crystal Blue.

Can I use Purple Rain instead of Crystal Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Purple Rain is a little darker, warmer than Crystal Blue.

Can I use Crystal Blue instead of Purple Rain?

The same difference applies the other way round: Crystal Blue is a little lighter, 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.