ΔE 16.6, loose: different enough that you should pick on purpose. Phthalo Cyanine Blue is a little darker, much more muted, warmer than Altdorf Guard Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Phthalo Cyanine Blue is a little darker, much more muted, warmer than Altdorf Guard Blue.
The same difference applies the other way round: Altdorf Guard Blue is a little lighter, much more saturated, cooler.
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 Phthalo Cyanine Blue on the colour wheel
All matches: Altdorf Guard Blue equivalents · Phthalo Cyanine Blue equivalents