Thanks for your query.
Well this appears to be a cosmetic problem . Skin colour and shades are largely governed by genetics. medical treatments exist for people having problems like chloasma
etc. If you have features of photosensitivity
along with increasing pigmentation or some rash
developing that needs to be evaluated but you didnt give any such mention in your history . For the cosmetic issue you should avoid direct exposure to sunlight by using umbrella or covering your face while in sun , use sunscreen lotions with high spf ( sun pprotection factor) , take a good nutitious diet , exercise regularly and lastly can use skin lightening creams and other options in consultation with dermatologist
Thank you again for choosin HCM for you query.