Lunar Eclipse vs Celestial Violet

How close are they?

ΔE 21.8, loose: different enough that you should pick on purpose. Celestial Violet is a little darker, cooler than Lunar Eclipse.

Can I use Celestial Violet instead of Lunar Eclipse?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Celestial Violet is a little darker, cooler than Lunar Eclipse.

Can I use Lunar Eclipse instead of Celestial Violet?

The same difference applies the other way round: Lunar Eclipse is a little lighter, 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.