Cultist Purple vs Ultramar Blue

How close are they?

ΔE 21.1, loose: different enough that you should pick on purpose. Ultramar Blue is noticeably darker, more muted, cooler than Cultist Purple.

Can I use Ultramar Blue instead of Cultist Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Ultramar Blue is noticeably darker, more muted, cooler than Cultist Purple.

Can I use Cultist Purple instead of Ultramar Blue?

The same difference applies the other way round: Cultist Purple is noticeably lighter, 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.