ΔE 19.5, loose: different enough that you should pick on purpose. Ultramarine is a little lighter, much more saturated, warmer than Altdorf Guard Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ultramarine is a little lighter, much more saturated, warmer than Altdorf Guard Blue.
The same difference applies the other way round: Altdorf Guard Blue is a little darker, much more muted, 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 Ultramarine on the colour wheel
All matches: Altdorf Guard Blue equivalents · Ultramarine equivalents