Final answer:
Bruises turn greenish in color because of the breakdown of heme into the green bile pigment biliverdin by the enzyme heme oxygenase within splenic macrophages, which is a stage in the healing of a bruise.
Step-by-step explanation:
When we see a bruise turning greenish, it is due to the metabolic process of heme, which is the non-iron component of hemoglobin. In injured tissues, such as those affected by a bruise, red blood cells (RBCs) are damaged and release heme. This heme is metabolically processed by heme oxygenase, an enzyme found within splenic macrophages, and is broken down into biliverdin. Biliverdin is a green bile pigment, which accounts for the green coloration observed in a healing bruise. As the bruise continues to heal, biliverdin is further reduced to bilirubin, which adds to the color changes of a bruise from red to purple to green and eventually to yellow.