What are all of the colors on the Assignment Progress screen?

The colors on the Assignment Progress screen give the instructor a visual representation of how students are doing on the assignment.

White - If the tile is white, the student has not even looked at the question.
Gray - The student has looked at the question, but has not yet attempted it.
Green - The student has attempted the question and gotten 100% on it
Yellow - The student has attempted the question and only got partial credit
Red - The student has attempted the question and did not get any part of the question correct.