All German units seem to have only white shields.
Plutarch mentions them when describing the Cimbrian cavalry (but does not mean Germans only used that color).
I think Cimbrian champion cavalry could still have only white shields.
Other units could have more varied colors (or just player color)
@Obelix I just didn't want to constantly shove that link on people's faces, but issues that have already been addressed in Thoughts on the Spartans/Athenians come up way too often