On the Assignment Report, the yellow box indicates the student pressed the "Show Answer" button for that question. If a student is stuck, pressing this button will provide the student with the answer so they can proceed with the assignment.
If the yellow box is paired with an answer, this indicates the student attempted the problem at least once before pressing the "Show Answer" button. If the yellow box is paired with "Answer Requested," the student pressed "Show Answer" right away. Remember that you can also dig in to analyze this student information on the Student Details Report as well.