Blood leaks into tissues under the skin and causes the black-and-blue color. As bruises (contusions) heal, usually within 2 to 4 weeks, they often turn colors, including purplish black, reddish blue, or yellowish green. Sometimes the area of the bruise spreads down the body in the direction of gravity. A bruise on a leg usually will take longer to heal than a bruise on the face or arms. Most bruises are not a cause for concern and will go away on their own. Home treatment may speed healing and relieve the swelling and soreness that often accompany bruises that are caused by injury.