ΔE 25.2, loose: different enough that you should pick on purpose. Dark Ultramarine is a little darker, cooler than Magecast Magenta.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 25). Dark Ultramarine is a little darker, cooler than Magecast Magenta.
The same difference applies the other way round: Magecast Magenta is a little lighter, 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 Magecast Magenta on the colour wheel See Dark Ultramarine on the colour wheel
All matches: Magecast Magenta equivalents · Dark Ultramarine equivalents