Under a student's profile there are a few colored label options you might see displayed.
"Open Assignments" Section:
Green = Completed on time / The student fully finished the assignment before the Due Date
Yellow and no Progress Bar = The student has not yet started to work on the assignment. You will notice there is no date in the "Last Activity" Column as well.
Yellow with tinted Progress Bar = The student has started the assignment, but has not yet gone through all of the slides in the lesson. You will see the date of their last activity on the lesson.
"Past Assignments" Section:
Red = The Assignment Due Date and Close Dates have passed, the assignment was never started, and the student no longer has access to work on the lesson.
Blue = The Assignment was completed, but was completed late / after the Due Date.
Green = The Assignment was completed, and completed on time / before the Due Date.