ΔE 12.0, fair: the same colour family, but not a straight swap. French Blue is a little darker, more muted than Altdorf Guard Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). French Blue is a little darker, more muted than Altdorf Guard Blue.
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.
See Altdorf Guard Blue on the colour wheel See French Blue on the colour wheel
All matches: Altdorf Guard Blue equivalents · French Blue equivalents