ΔE 26.5, loose: different enough that you should pick on purpose. Nightshade Purple is noticeably darker, more muted, warmer than Highlord Blue.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Nightshade Purple is noticeably darker, more muted, warmer than Highlord Blue.
The same difference applies the other way round: Highlord Blue is noticeably lighter, more saturated, 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 Highlord Blue on the colour wheel See Nightshade Purple on the colour wheel
All matches: Highlord Blue equivalents · Nightshade Purple equivalents