Ultramarine Blue vs Prussian Blue

How close are they?

ΔE 15.3, loose: different enough that you should pick on purpose. Prussian Blue is noticeably darker, more muted than Ultramarine Blue.

Can I use Prussian Blue instead of Ultramarine Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Prussian Blue is noticeably darker, more muted than Ultramarine Blue.

Can I use Ultramarine Blue instead of Prussian Blue?

The same difference applies the other way round: Ultramarine Blue is noticeably lighter, 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.