Prussian Blue vs Altdorf Guard Blue

How close are they?

ΔE 8.1, close: similar, with a visible difference side by side. Altdorf Guard Blue is more saturated than Prussian Blue.

Can I use Altdorf Guard Blue instead of Prussian Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 8). Altdorf Guard Blue is more saturated than Prussian Blue.

Can I use Prussian Blue instead of Altdorf Guard Blue?

The same difference applies the other way round: Prussian Blue is 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.