ΔE 11.1, fair: the same colour family, but not a straight swap. Sapphire Blue is more muted, cooler than Ultramarines Blue. They are different kinds of paint: Citadel Ultramarines Blue is a one-coat contrast-style paint and Reaper Sapphire Blue is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 11). Sapphire Blue is more muted, cooler than Ultramarines Blue. They are different kinds of paint.
The same difference applies the other way round: Ultramarines Blue is more saturated, 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 Ultramarines Blue on the colour wheel See Sapphire Blue on the colour wheel
All matches: Ultramarines Blue equivalents · Sapphire Blue equivalents