Bright Violet vs Light Violet

How close are they?

ΔE 15.7, loose: different enough that you should pick on purpose. Light Violet is noticeably lighter, more saturated than Bright Violet.

Can I use Light Violet instead of Bright Violet?

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

Can I use Bright Violet instead of Light Violet?

The same difference applies the other way round: Bright Violet 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.