Frostveil vs Altdorf Guard Blue

How close are they?

ΔE 6.8, close: similar, with a visible difference side by side. Altdorf Guard Blue is a little darker, more muted than Frostveil.

Can I use Altdorf Guard Blue instead of Frostveil?

Usually, if you are not matching paint already on the model. Close match (ΔE 7). Altdorf Guard Blue is a little darker, more muted than Frostveil.

Can I use Frostveil instead of Altdorf Guard Blue?

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

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.