Deep Violet vs Coven Purple

How close are they?

ΔE 17.5, loose: different enough that you should pick on purpose. Coven Purple is noticeably darker, more muted than Deep Violet.

Can I use Coven Purple instead of Deep Violet?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 18). Coven Purple is noticeably darker, more muted than Deep Violet.

Can I use Deep Violet instead of Coven Purple?

The same difference applies the other way round: Deep Violet is noticeably lighter, 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.