WWI German Lilac vs Forbidden Fruit

How close are they?

ΔE 15.2, loose: different enough that you should pick on purpose. Forbidden Fruit is more saturated, warmer than WWI German Lilac.

Can I use Forbidden Fruit instead of WWI German Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 15). Forbidden Fruit is more saturated, warmer than WWI German Lilac.

Can I use WWI German Lilac instead of Forbidden Fruit?

The same difference applies the other way round: WWI German Lilac is more muted, 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.