WWI German Lilac vs Light Violet

How close are they?

ΔE 16.5, loose: different enough that you should pick on purpose. Light Violet is a little darker, much more saturated, cooler than WWI German Lilac.

Can I use Light Violet instead of WWI German Lilac?

Not as a straight swap; see the closer alternatives below. Loose match (ΔE 16). Light Violet is a little darker, much more saturated, cooler than WWI German Lilac.

Can I use WWI German Lilac instead of Light Violet?

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