ΔE 13.4, fair: the same colour family, but not a straight swap. Amethyst Purple is a little darker, more saturated, cooler than Cloud Nine.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 13). Amethyst Purple is a little darker, more saturated, cooler than Cloud Nine.
The same difference applies the other way round: Cloud Nine is a little lighter, more muted, 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 Amethyst Purple on the colour wheel
All matches: Cloud Nine equivalents · Amethyst Purple equivalents