Your list shows due dates for a certain Name/Job title...
Now, there are couple of things you would have to to check before assigning colors:
Are all the dates only visible when the due date is below 30? (judging by your question, ofcourse not but, still) if that is the case then all list items would be of the same color and it doesn't matter what color it is, meaning black(grey) is the color you go for on white background.
Who uses the list? This can also decide the color for ex- If certain person or designation sees the list and s/he is the only person who performs the actions then you can arrange the list as per due date and no need to mark a different color to the list.
And, if the users themselves use the list than they have to focus on only one item (list should be searchable) and no different color is needed.
What is the difference between a person with due 30 days and person with due 31 days? Is there? like you can only take the action when due date is whithin 30 dates? or something like that... if not, then color differentiation doesn't matter and you can use the same color for all.
And if there is a case I've missed and/or you want to use color is case 2(not needed really, use bold) or Case 3 than Green is the color you go for. Reason- Blue is reserved by convention for links, red is error/warning so, you're left with Green only.