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There are various reasons as to why you might be getting dark...may be sun exposure or tanning
,genetic,some chemicals etc.The best thing to begin with is to start using a good sunscreen at least three times a day.At night you can use glycolic acid
creams or hydroquinone
based creams which can lighten up your color.Consult a skin specialist for a chemical peel which can also help lighten up the color.