This is common when people comment on others for being dark complexion and we definitely cannot take any action as it is a mindset thing. People who are literate but do not have the knowledge to treat others are not worthy. You should only focus on your work and never revert to them. The best practice you can do is to ignore and avoid such people when they are around you. This is not law firm advice but handle this criticism with a smile and thank them. 
Being complexioned is not a choice and you should not feel ashamed of it. Praise your complexion with pride.