Purple Swarm vs Dark Purple

How close are they?

ΔE 21.5, loose: different enough that you should pick on purpose. Dark Purple is noticeably darker, much more muted than Purple Swarm.

Can I use Dark Purple instead of Purple Swarm?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Dark Purple is noticeably darker, much more muted than Purple Swarm.

Can I use Purple Swarm instead of Dark Purple?

The same difference applies the other way round: Purple Swarm is noticeably lighter, much more saturated.

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.