Ultramarines Blue vs Arcane Purple

How close are they?

ΔE 26.7, loose: different enough that you should pick on purpose. Arcane Purple is noticeably darker, more muted, warmer than Ultramarines Blue.

Can I use Arcane Purple instead of Ultramarines Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Arcane Purple is noticeably darker, more muted, warmer than Ultramarines Blue.

Can I use Ultramarines Blue instead of Arcane Purple?

The same difference applies the other way round: Ultramarines Blue is noticeably lighter, more saturated, cooler.

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.