Deep Blue vs Crystal Blue

How close are they?

ΔE 17.7, loose: different enough that you should pick on purpose. Crystal Blue is a little darker, more muted, cooler than Deep Blue.

Can I use Crystal Blue instead of Deep Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Crystal Blue is a little darker, more muted, cooler than Deep Blue.

Can I use Deep Blue instead of Crystal Blue?

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