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kish,color of skin depends mostly on your genes and ethnic origin.If not this then it can be induced also due to various reasons like excess exposure to sunlight leading to tanning
, exposure to some allergic chemicals like those present in hair dye or face creams .Certain element on oral ingestion also might cause pigmentation .So ,a detailed history is very important here.
As far as treatment is concerned ,the most important thing would be avoidance of the particular chemical or cream or sunlight as the case might be. Also there is a need if use of good sunscreen daily thrice.There are certain creams which help reducing the dark color are hydroquinone
cream,glycolic cream ,kojic acid
Even procedures like chemical peel can be done to improve complexion.
But if your complexion is genetically dark then the possibility of improvement is very bleak.
You should consult a experienced dermatologist
to rule out above causes and find a good treatment if possible.