Talassar Blue vs Dark Purple

How close are they?

ΔE 39.3, loose: different enough that you should pick on purpose. Dark Purple is noticeably darker, warmer than Talassar Blue.

Can I use Dark Purple instead of Talassar Blue?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 39). Dark Purple is noticeably darker, warmer than Talassar Blue.

Can I use Talassar Blue instead of Dark Purple?

The same difference applies the other way round: Talassar 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.