Blue Base vs Light Violet

How close are they?

ΔE 26.7, loose: different enough that you should pick on purpose. Light Violet is noticeably lighter, warmer than Blue Base.

Can I use Light Violet instead of Blue Base?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 27). Light Violet is noticeably lighter, warmer than Blue Base.

Can I use Blue Base instead of Light Violet?

The same difference applies the other way round: Blue Base is noticeably darker, 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.