Forbidden Fruit vs Purple

How close are they?

ΔE 21.6, loose: different enough that you should pick on purpose. Purple is noticeably darker, cooler than Forbidden Fruit.

Can I use Purple instead of Forbidden Fruit?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Purple is noticeably darker, cooler than Forbidden Fruit.

Can I use Forbidden Fruit instead of Purple?

The same difference applies the other way round: Forbidden Fruit is noticeably lighter, warmer.

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.