ΔE 16.6, loose: different enough that you should pick on purpose. Grey Seer is a little lighter, much more muted than WWI German Lilac.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Grey Seer is a little lighter, much more muted than WWI German Lilac.
The same difference applies the other way round: WWI German Lilac is a little darker, 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.
See WWI German Lilac on the colour wheel See Grey Seer on the colour wheel
All matches: WWI German Lilac equivalents · Grey Seer equivalents