ΔE 35.6, loose: different enough that you should pick on purpose. Royal Purple is a little darker, much more saturated, warmer than Celestium Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 36). Royal Purple is a little darker, much more saturated, warmer than Celestium Blue.
The same difference applies the other way round: Celestium Blue is a little lighter, much more muted, cooler.
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 Celestium Blue on the colour wheel See Royal Purple on the colour wheel
All matches: Celestium Blue equivalents · Royal Purple equivalents