Darkening of face. Preventive measures.

Thanks for the query. The description provided by you is insufficient to arrive at a conclusive diagnosis and treatment plan. You may have facial melanosis which could be melasma compounded by acanthosis nigricans. These conditions have a strong relation to one's internal health and it needs to be evaluated as often quoted - Skin is the mirror of internal health. Attempting treatment without understanding the pathology will be futile and make the course longer. Your dermatologist will decide a treatment plan for you after a consultation. Sun protection is mandatory and will require usage of Sunscreens. Internal health needs to be fixed based on blood chemistry reports. Apart from active pharmaceutical medicines, a healthy diet and lifestyle is mandatory to achieve results

You need to frame your query more appropriately to get a precise recommendations, A clear photograph would have been helpful.

Your dermatologist will decide a treatment plan for you after a consultation.