Something like this ? :
Dim d1 As DateTime = DateTime.Now
Dim dueDate As DateTime = .....
Dim result = ""
'If overdue for one day or more'
If (d1 - dueDate).Days >= 1 Then
result = "red"
'If 3 days or more to due date'
ElseIf (dueDate - d1).Days <= 3 Then
result = "yellow"
Else : result = "green"
End If