ΔE 11.5, fair: the same colour family, but not a straight swap. UK Azure Blue is a little lighter, more muted than Ultramar Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). UK Azure Blue is a little lighter, more muted than Ultramar Blue.
The same difference applies the other way round: Ultramar Blue is a little darker, more saturated.
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 UK Azure Blue on the colour wheel
All matches: Ultramar Blue equivalents · UK Azure Blue equivalents