Pigmentation in the neck may be due to various reasons...? Acanthosis nigricans - it comes with increased insulin resistance -secondarly due to increased body wieght, hypothyroidism, pcos ..any other metabolic diseases. If its normal pigmentation due to sun means use Golite cream in the morning.
Night -Tanfree lotion both hands & neck ..tanfree lotion contain glylocic acid please use very little and spread it evenly.
Next Steps
kindly confirm with the near by dermatologist for further treament & evaluation if the creams prescribed is not working.