ΔE 11.7, fair: the same colour family, but not a straight swap. Ultramarine Blue is a little darker, warmer than Amarth Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Ultramarine Blue is a little darker, warmer than Amarth Blue.
The same difference applies the other way round: Amarth Blue is a little lighter, 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 Amarth Blue on the colour wheel See Ultramarine Blue on the colour wheel
All matches: Amarth Blue equivalents · Ultramarine Blue equivalents