The color of the arm may be caused by the following conditions: 1, after a long period of sun exposure arm color will become black, but this darkening generally exists only in the exposed skin, mainly due to radiation makes melanin deposition caused by. 2, due to collision and other reasons caused by the arm local bruising, this situation is generally pressed bruised parts will have a painful feeling, and after a period of time the bruised parts can be self-relieved. 3, when there is a rash, dermatitis and other conditions local arm pigmentation will also appear, but in this case will generally be accompanied by itching sensation.