ΔE 14.1, fair: the same colour family, but not a straight swap. Imperial Blue is noticeably lighter, more saturated, cooler than Ultramar Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Imperial Blue is noticeably lighter, more saturated, cooler than Ultramar Blue.
The same difference applies the other way round: Ultramar Blue is noticeably darker, more muted, 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 Ultramar Blue on the colour wheel See Imperial Blue on the colour wheel
All matches: Ultramar Blue equivalents · Imperial Blue equivalents