Blue Violet vs Crystal Blue

How close are they?

ΔE 27.3, loose: different enough that you should pick on purpose. Crystal Blue is a little darker, much more saturated, cooler than Blue Violet.

Can I use Crystal Blue instead of Blue Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Crystal Blue is a little darker, much more saturated, cooler than Blue Violet.

Can I use Blue Violet instead of Crystal Blue?

The same difference applies the other way round: Blue Violet is a little lighter, much more muted, warmer.

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.