Eldandil Violet vs Flat Blue

How close are they?

ΔE 16.1, loose: different enough that you should pick on purpose. Flat Blue is a little lighter, cooler than Eldandil Violet.

Can I use Flat Blue instead of Eldandil Violet?

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

Can I use Eldandil Violet instead of Flat Blue?

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