Royal Cloak vs Dark Prussian Blue

How close are they?

ΔE 18.8, loose: different enough that you should pick on purpose. Dark Prussian Blue is noticeably darker, more saturated, cooler than Royal Cloak.

Can I use Dark Prussian Blue instead of Royal Cloak?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Dark Prussian Blue is noticeably darker, more saturated, cooler than Royal Cloak.

Can I use Royal Cloak instead of Dark Prussian Blue?

The same difference applies the other way round: Royal Cloak is noticeably lighter, more muted, warmer.

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.