Cerulean Blue Deep vs Violet

How close are they?

ΔE 15.9, loose: different enough that you should pick on purpose. Violet is a little darker, more muted, warmer than Cerulean Blue Deep.

Can I use Violet instead of Cerulean Blue Deep?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Violet is a little darker, more muted, warmer than Cerulean Blue Deep.

Can I use Cerulean Blue Deep instead of Violet?

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