It depends on what you want. You can make the victory messages red and with Soviet flags, after all, this is a Soviet campaign.
For the 1941 campaign I will use this system:
Political events of the Eastern Front - either Soviet or Axis flags (or a historical photo) in either Soviet or Axis colors, depending of who is mentioned in the message.
The message on arrival of the Polizei Division also has Axis colors with a German flag.
(because it is not an Army unit)
Military events, like destroying bridges or sending reinforcements - green neutral (color of the field uniform) with either Soviet or Axis flags, depending on who blows bridges or sends reinforcements.
Military events in other theaters like North Africa - Allied green background with either Allied flags or historical photos if they are in data base.
Ad you said, it is all subjective. I would say, if you take Minsk, that would be a message in Soviet colors with a Soviet flag. You can decide what to do with other messages like this one, they could also be in the Soviet color.