When you have made an Assignment where:

1) In Group submission settings, "Students submit in groups is set as "Yes" and "Require all group members submit" is set as "No"

2) In Activity completion settings, the only ticked option is "Student must submit to this activity to complete it "

...then a student makes a submission, but the Completion progress bar does not show a green bar - only yellow. Neither the submitter or the other group members get the green bar.

The colors work, however, when you also turn on "Student must receive a grade to complete this activity" and have a teacher grade the submission.