A patch of dark-colored spots on the arm may be related to hyperpigmentation, sun spots, lichen planus and so on. 1. Hyperpigmentation: it may be due to trauma, local inflammation, allergic rash and other diseases. It leads to an increase in the amount of local pigmentation, which may appear as brown or black spots on the local skin. 2. Sunburn: it may be the arm after intense sun exposure, the sunburned skin becomes red and swollen and other reactions. After the condition subsides, flaky scales or dark brown spots may appear on the sunburned area. 3. Lichen planus: This is an abnormal pigmentation of the arm that appears as spots or plaques due to an infection by the bacteria Malassezia. Patches such as brown or light brown may appear and can sometimes fuse into patches. A patch of dark-colored spots on the arm can also be seen in other causes, it is recommended to consult a doctor in a timely manner, follow the doctor’s instructions for relevant examinations, with the help of the doctor to clarify the nature, and targeted treatment or therapy.