Forbidden Fruit vs N6 Neutral Gray

How close are they?

ΔE 32.4, loose: different enough that you should pick on purpose. N6 Neutral Gray is a little darker, much more muted than Forbidden Fruit.

Can I use N6 Neutral Gray instead of Forbidden Fruit?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 32). N6 Neutral Gray is a little darker, much more muted than Forbidden Fruit.

Can I use Forbidden Fruit instead of N6 Neutral Gray?

The same difference applies the other way round: Forbidden Fruit is a little 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.