A bruise appears as a greenish spot in the skin because

A bruise is a type of hematoma of tissue in which capillaries are damaged by trauma, allowing blood to leak into the surrounding tissues. As a result, tenderness and discolouration under skin occur. A bruise is healed when the tissue absorbs that leaked blood and the appearance of a bruise will change depending on where an when it is formed. Greenish colour appears after red and purple and it is a sign that the haemoglobin is breaking down. Greenish colour also means the healing process has begun.