ΔE 21.4, loose: different enough that you should pick on purpose. Celestial Violet is noticeably lighter, much more muted, cooler than Cloud Nine.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Celestial Violet is noticeably lighter, much more muted, cooler than Cloud Nine.
The same difference applies the other way round: Cloud Nine is noticeably darker, much more saturated, 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.
See Cloud Nine on the colour wheel See Celestial Violet on the colour wheel
All matches: Cloud Nine equivalents · Celestial Violet equivalents