ΔE 16.2, loose: different enough that you should pick on purpose. Prussian Blue is a little darker, much more saturated than Faust Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Prussian Blue is a little darker, much more saturated than Faust Blue.
The same difference applies the other way round: Faust Blue is a little lighter, much more muted.
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 Faust Blue on the colour wheel See Prussian Blue on the colour wheel
All matches: Faust Blue equivalents · Prussian Blue equivalents