Blue Violet vs Eidolon Purple Clear

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Eidolon Purple Clear is noticeably darker, warmer than Blue Violet.

Can I use Eidolon Purple Clear instead of Blue Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Eidolon Purple Clear is noticeably darker, warmer than Blue Violet.

Can I use Blue Violet instead of Eidolon Purple Clear?

The same difference applies the other way round: Blue Violet is noticeably lighter, 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.