What causes a wound scab to turn yellow?

Yellowing of a wound's scab is caused by secretions or pus. During the healing process, wounds often produce a yellowish fluid that, when dried and coagulated, forms a yellowish scab, which generally indicates the wound has healed. If there is no local redness, swelling, pain, pus discharge, or large amounts of yellow fluid, this is a normal phenomenon. However, you should avoid pressing on the wound; allow it to heal naturally.