ΔE 12.9, fair: the same colour family, but not a straight swap. Ultramarine Highlight is noticeably lighter, cooler than Altdorf Guard Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Ultramarine Highlight is noticeably lighter, cooler than Altdorf Guard Blue.
The same difference applies the other way round: Altdorf Guard Blue is noticeably darker, warmer.
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 Highlight on the colour wheel
All matches: Altdorf Guard Blue equivalents · Ultramarine Highlight equivalents