The main reason for pigmentation in the exposed part is sun exposure.
Apart from using sunscreen, use Sun protection like an umbrella, full sleeve clothes, long sleeve driving gloves and covered footwear.
And do apply a generous amount of sunscreen apart from your face even on the exposed body parts.
Next Steps
Apart from this medicated creams are there to reduce pigmentation and in clinic procedures to reduce the pigmentation. For that I always advise an in person consultation for the best results.