Brown spots on a cat's nipples can be a sign of a few different conditions. The most common cause of brown spots on a cat's nipples is usually just normal pigmentation. Just like with humans, cats can have variations in the color of their skin, and this can include the nipples. However, if the spots appear suddenly or are accompanied by other symptoms such as itching, redness, or discharge, it could be a sign of a more serious condition. For example, if the spots are raised, itchy, or bleeding, it could be a sign of a skin infection or a tumor. If you are concerned about the brown spots on your cat's nipples, it is important to have them examined by a veterinarian to determine the underlying cause and the best course of treatment.