ΔE 15.2, loose: different enough that you should pick on purpose. Celestial Violet is much more saturated than Emperor's Purple.
Can I use Celestial Violet instead of Emperor's Purple?
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Celestial Violet is much more saturated than Emperor's Purple.
Can I use Emperor's Purple instead of Celestial Violet?
The same difference applies the other way round: Emperor's Purple is much more muted.
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.