Final answer:
Postmortem staining is a change in tissue color due to hemoglobin breakdown products, but the specific cause in the context of postmortem changes is tissue decomposition.
Step-by-step explanation:
Postmortem staining is an extravascular change in the color of tissue due to the breakdown pigments formed from the destruction of hemoglobin. These pigments, such as biliverdin and bilirubin, cause discoloration in various situations, such as bruising or jaundice. However, the final answer to the student's question is that postmortem staining is caused by decomposition, which is associated with the breakdown of hemoglobin and other tissues after death, leading to the characteristic color changes.