For accessibility reasons color shouldn't be the only way to show information for the user. In Activity completion report unmarked by teacher status is shown using only a red border.

Steps to reproduce:

1. Create a course C1
2. Add an assignment A1 and set the Make a submission as completion criteria
3. Enrol user S1
4. Go to Report / Activity completion
5. Mark the activity as complete (as a teacher)
6. Unmark it