The causes of localized purple skin after foot sprain are as follows. If the foot sprain is mild, it is usually due to injury to the joint capsule and other tissues, and the rupture of small local blood vessels, causing subcutaneous bruising, which can be manifested as localized bruising after hemoglobin is decomposed. In addition, if the injury is relatively serious, a sprain of the medial collateral ligament or lateral collateral ligament of the ankle joint can also cause damage to the surrounding blood vessels, which can manifest as a local purple phenomenon. In severe cases, fractures of the ankle joint, such as fractures of the inner ankle and the outer ankle, can lead to bleeding at the fracture end, and the blood will leak out into the subcutaneous tissue, which can also form a local purple phenomenon. Therefore, no matter what kind of structural damage is done after foot sprain, local purple color may occur.