ΔE 11.6, fair: the same colour family, but not a straight swap. Flat Blue is a little lighter, warmer than Ultramarine Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 12). Flat Blue is a little lighter, warmer than Ultramarine Blue.
The same difference applies the other way round: Ultramarine Blue is a little darker, 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 Ultramarine Blue on the colour wheel See Flat Blue on the colour wheel
All matches: Ultramarine Blue equivalents · Flat Blue equivalents