ΔE 21.1, loose: different enough that you should pick on purpose. Dark Prussian Blue is noticeably darker, much more muted, warmer than Cerulean Blue Deep.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dark Prussian Blue is noticeably darker, much more muted, warmer than Cerulean Blue Deep.
The same difference applies the other way round: Cerulean Blue Deep is noticeably lighter, much more saturated, cooler.
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.
See Cerulean Blue Deep on the colour wheel See Dark Prussian Blue on the colour wheel
All matches: Cerulean Blue Deep equivalents · Dark Prussian Blue equivalents