Altdorf Guard Blue vs Cerulean Blue Deep

How close are they?

ΔE 8.7, close: similar, with a visible difference side by side. Cerulean Blue Deep is a little darker, more muted than Altdorf Guard Blue.

Can I use Cerulean Blue Deep instead of Altdorf Guard Blue?

Usually, if you are not matching paint already on the model. Close match (ΔE 9). Cerulean Blue Deep is a little darker, more muted than Altdorf Guard Blue.

Can I use Altdorf Guard Blue instead of Cerulean Blue Deep?

The same difference applies the other way round: Altdorf Guard Blue 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.