ΔE 20.2, loose: different enough that you should pick on purpose. Ultramarine is noticeably lighter, much more saturated than Macragge Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Ultramarine is noticeably lighter, much more saturated than Macragge Blue.
The same difference applies the other way round: Macragge Blue is noticeably darker, much 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 Macragge Blue on the colour wheel See Ultramarine on the colour wheel
All matches: Macragge Blue equivalents · Ultramarine equivalents