Eldandil Violet vs Blue Violet

How close are they?

ΔE 15.6, loose: different enough that you should pick on purpose. Blue Violet is a little lighter, more saturated, cooler than Eldandil Violet.

Can I use Blue Violet instead of Eldandil Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Blue Violet is a little lighter, more saturated, cooler than Eldandil Violet.

Can I use Eldandil Violet instead of Blue Violet?

The same difference applies the other way round: Eldandil Violet is a little darker, 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.