Highlord Blue vs Dark Purple

How close are they?

ΔE 31.9, loose: different enough that you should pick on purpose. Dark Purple is noticeably darker, warmer than Highlord Blue.

Can I use Dark Purple instead of Highlord Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 32). Dark Purple is noticeably darker, warmer than Highlord Blue.

Can I use Highlord Blue instead of Dark Purple?

The same difference applies the other way round: Highlord Blue is noticeably lighter, 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.