Cerulean Blue Deep vs Ultramarine Blue

How close are they?

ΔE 22.4, loose: different enough that you should pick on purpose. Ultramarine Blue is noticeably lighter, more saturated than Cerulean Blue Deep.

Can I use Ultramarine Blue instead of Cerulean Blue Deep?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Ultramarine Blue is noticeably lighter, more saturated than Cerulean Blue Deep.

Can I use Cerulean Blue Deep instead of Ultramarine Blue?

The same difference applies the other way round: Cerulean Blue Deep is noticeably darker, more muted.

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.