The easiest way to do this is using a python code block in the field calculator in ArcMap, however if you're wanting to do it in a script, I would create an arcpy.UpdateCursor(), then calculate the value for Label and add it row by row.
feature_class = r"path\to\class"
cursor = arcpy.UpdateCursor(feature_class)
for row in cursor:
label = ""
difference = row.getValue('Difference')
if difference == 0:
label += "Label is 0"
elif difference > 0.30:
// ...
row.setValue("Label", label)
cursor.updateRow(row)