Darkness like dirt on the back of the neck is not necessarily a disease, and may be caused by physiological and pathological reasons, among which pathological reasons include the possibility of acanthosis nigricans and lichen planus. 1. Physiological reasons: Simple local skin color changes in the neck, such as excessive sun exposure, or cleaning without paying attention to the skin at the back of the neck, may lead to darkening of the skin color. 2. Pathologic causes: (1) Acanthosis nigricans: it may be related to heredity, endocrine, drugs, tumors and other factors, and it mostly occurs in the folds of the skin. Due to hyperkeratosis of the epidermis, papillomatous hyperplasia and increased pigmentation at the base. It is often manifested as gray-brown or black skin at the back of the neck, which looks like velvet when touched. (2) lichen planus: it is due to the Malassezia bacteria that reside in human body, transforming into pathogenic bacteria under certain conditions, invading the cuticle of the skin and causing brown skin lesions. When the focus of infection is at the back of the neck, it presents with darkened skin color at the back of the neck. If the skin on the back of the neck turns black, it is recommended to consult a doctor in time and follow the doctor’s instructions for further examination to clarify the cause and carry out targeted treatment.