ΔE 14.5, fair: the same colour family, but not a straight swap. Blue Violet is noticeably lighter than Phthalo Blue RS.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 14). Blue Violet is noticeably lighter than Phthalo Blue RS.
The same difference applies the other way round: Phthalo Blue RS is noticeably darker.
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 Phthalo Blue RS on the colour wheel See Blue Violet on the colour wheel
All matches: Phthalo Blue RS equivalents · Blue Violet equivalents