ΔE 10.1, fair: the same colour family, but not a straight swap. German Grey is a little darker, more muted than Darkness.
Only where an exact match doesn’t matter, such as a new unit or a highlight. Fair match (ΔE 10). German Grey is a little darker, more muted than Darkness.
The same difference applies the other way round: Darkness is a little lighter, 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 Darkness on the colour wheel See German Grey on the colour wheel
All matches: Darkness equivalents · German Grey equivalents