ΔE 14.0, fair: the same colour family, but not a straight swap. Ultramarine is more saturated, warmer than Viking Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Ultramarine is more saturated, warmer than Viking Blue.
The same difference applies the other way round: Viking Blue is 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 Viking Blue on the colour wheel See Ultramarine on the colour wheel
All matches: Viking Blue equivalents · Ultramarine equivalents