Ultramarine Violet vs Light Violet

How close are they?

ΔE 20.1, loose: different enough that you should pick on purpose. Light Violet is much more muted, warmer than Ultramarine Violet.

Can I use Light Violet instead of Ultramarine Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Light Violet is much more muted, warmer than Ultramarine Violet.

Can I use Ultramarine Violet instead of Light Violet?

The same difference applies the other way round: Ultramarine Violet is much 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.