Purple Alchemy vs Deep Purple

How close are they?

ΔE 25.9, loose: different enough that you should pick on purpose. Deep Purple is noticeably darker, much more muted than Purple Alchemy.

Can I use Deep Purple instead of Purple Alchemy?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Deep Purple is noticeably darker, much more muted than Purple Alchemy.

Can I use Purple Alchemy instead of Deep Purple?

The same difference applies the other way round: Purple Alchemy 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.