Cultist Purple vs Ultramarine Violet

How close are they?

ΔE 16.4, loose: different enough that you should pick on purpose. Ultramarine Violet is noticeably lighter, more saturated than Cultist Purple.

Can I use Ultramarine Violet instead of Cultist Purple?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Ultramarine Violet is noticeably lighter, more saturated than Cultist Purple.

Can I use Cultist Purple instead of Ultramarine Violet?

The same difference applies the other way round: Cultist Purple 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.