ΔE 23.1, loose: different enough that you should pick on purpose. Ultramarine Highlight is noticeably lighter, more muted, cooler than Beowulf Blue. They are different kinds of paint: Army Painter Beowulf Blue is a one-coat contrast-style paint and Reaper Ultramarine Highlight 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 23). Ultramarine Highlight is noticeably lighter, more muted, cooler than Beowulf Blue. They are different kinds of paint.
The same difference applies the other way round: Beowulf Blue is noticeably darker, 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 Beowulf Blue on the colour wheel See Ultramarine Highlight on the colour wheel
All matches: Beowulf Blue equivalents · Ultramarine Highlight equivalents