ΔE 12.9, fair: the same colour family, but not a straight swap. Ultramarine Highlight is noticeably lighter, more saturated than Regal Blue.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Ultramarine Highlight is noticeably lighter, more saturated than Regal Blue.
The same difference applies the other way round: Regal Blue is noticeably darker, more muted.
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 Regal Blue on the colour wheel See Ultramarine Highlight on the colour wheel
All matches: Regal Blue equivalents · Ultramarine Highlight equivalents