ΔE 15.5, loose: different enough that you should pick on purpose. Translucent Purple is a little lighter, more saturated, warmer than Altdorf Guard Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Translucent Purple is a little lighter, more saturated, warmer than Altdorf Guard Blue.
The same difference applies the other way round: Altdorf Guard Blue is a little darker, 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 Translucent Purple on the colour wheel
All matches: Altdorf Guard Blue equivalents · Translucent Purple equivalents