Blue Violet vs Ultramarine Blue

How close are they?

ΔE 18.3, loose: different enough that you should pick on purpose. Ultramarine Blue is more saturated, cooler than Blue Violet.

Can I use Ultramarine Blue instead of Blue Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Ultramarine Blue is more saturated, cooler than Blue Violet.

Can I use Blue Violet instead of Ultramarine Blue?

The same difference applies the other way round: Blue Violet is more muted, 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.