Frostveil vs Prussian Blue

How close are they?

ΔE 20.1, loose: different enough that you should pick on purpose. Prussian Blue is a little darker, much more muted, cooler than Frostveil.

Can I use Prussian Blue instead of Frostveil?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Prussian Blue is a little darker, much more muted, cooler than Frostveil.

Can I use Frostveil instead of Prussian Blue?

The same difference applies the other way round: Frostveil is a little lighter, much more saturated, 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.