To treat a sun tan and dark neck, start with daily broad-spectrum sunscreen (SPF 30+) to prevent further darkening. For topical creams, look for generic active ingredients like Vitamin C, Niacinamide, Kojic acid, Alpha-Arbutin, or Glycolic acid, which help gently exfoliate and lighten the skin. Prescription retinoids can also boost skin cell turnover. For faster results, dermatological procedures like mild chemical peels, microdermabrasion, or laser therapy are very effective. This is a very common, highly treatable condition. I recommend consulting a dermatologist to tailor these options safely to your skin type.