ΔE 27.6, loose: different enough that you should pick on purpose. Indigo Blue is a little darker, much more muted, cooler than Ultramarines Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 28). Indigo Blue is a little darker, much more muted, cooler than Ultramarines Blue.
The same difference applies the other way round: Ultramarines Blue is a little lighter, much 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 Indigo Blue on the colour wheel
All matches: Ultramarines Blue equivalents · Indigo Blue equivalents