Prussian Blue vs Dark Blue

How close are they?

ΔE 15.1, loose: different enough that you should pick on purpose. Dark Blue is a little lighter, much more muted than Prussian Blue.

Can I use Dark Blue instead of Prussian Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Dark Blue is a little lighter, much more muted than Prussian Blue.

Can I use Prussian Blue instead of Dark Blue?

The same difference applies the other way round: Prussian Blue is a little darker, 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.