ΔE 19.4, loose: different enough that you should pick on purpose. Indigo Sky is noticeably lighter, much more muted than Ultramarines Blue. They are different kinds of paint: Citadel Ultramarines Blue is a one-coat contrast-style paint and Reaper Indigo Sky is an opaque base/layer paint, so they won’t behave the same on the model even where the colour is close.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 19). Indigo Sky is noticeably lighter, much more muted than Ultramarines Blue. They are different kinds of paint.
The same difference applies the other way round: Ultramarines Blue is noticeably darker, much 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 Ultramarines Blue on the colour wheel See Indigo Sky on the colour wheel
All matches: Ultramarines Blue equivalents · Indigo Sky equivalents