Altdorf Guard Blue vs Prussian Blue

How close are they?

ΔE 16.3, loose: different enough that you should pick on purpose. Prussian Blue is noticeably darker, more muted, warmer than Altdorf Guard Blue.

Can I use Prussian Blue instead of Altdorf Guard Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Prussian Blue is noticeably darker, more muted, warmer than Altdorf Guard Blue.

Can I use Altdorf Guard Blue instead of Prussian Blue?

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