Pressure sores are classified as stage I through stage IV, with stage I being the least severe and stage IV being the most severe, notes MedlinePlus. A pressure sore can't be classified if the base of the sore is covered with tan, brown, yellow or green dead skin.
Stage I pressure sores don't have broken skin, notes Mayo Clinic. In some cases, the temperature of the wound is different from the surrounding skin, and the area is usually discolored. Stage II pressure sores often appear shallow because the uppermost layer of skin and part of the layer beneath are missing. Blisters may form on the sore.
Stage III pressure sores are often deep with some fat exposed, states Mayo Clinic. The bottom of the sore may be yellow because of dead tissue. Stage IV pressure sores likely have exposed tendons, muscle or bone. The bottom of the sore may appear crusty and dark or yellow because of dead tissue.