Deep Blue vs Ultramarine Blue

How close are they?

ΔE 19.0, loose: different enough that you should pick on purpose. Ultramarine Blue is a little darker, much more muted than Deep Blue.

Can I use Ultramarine Blue instead of Deep Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Ultramarine Blue is a little darker, much more muted than Deep Blue.

Can I use Deep Blue instead of Ultramarine Blue?

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

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.