Livor mortis, often referred to as postmortem lividity, is a purplish discoloration of the skin following death. The discoloration is a result of blood pooling in the lower parts of the body. You see, once the heart stops pumping, gravity will force the blood to seep into these lower areas. Lividity will not form in areas of the body pressing hard against the floor because the pressure against the capillaries won’t allow the blood to settle. Instead, these areas will result in a white coloration called blanching. You can mimic this effect by gripping your forearm tightly with your opposite hand. Squeeze tightly for a few seconds and then release. You should see a whitish coloring where your fingers were.
Lividity is important to the investigator for two reasons. First, it can provide investigators with a general understanding of the time since death. These estimates are influenced by things such as temperature, body mass, conditions at the crime scene, and other factors. Suffice it to say I’m only giving you very general time frames but for your novel they should be fine. Lividity is generally seen within the first 1-2 hours following death. It will typically become fully developed within 3-4 hours but could still be altered if the body position changed. After 12 hours the lividity becomes fixed meaning it will not change.
As I have just alluded, there is another important reason for investigators to note postmortem lividity. It can indicate a change in body position. If, for example, your detective finds a body face up with the purplish color face up as well; it’s a clear indication that the body was turned over several hours after death. Investigators can also use the areas of blanching to help reconstruct the original position of the body. Blanching might even indicate another location or crime scene. Hypothetically, if you were to see 4″ wide slots of blanching separated by thin purple lines that may indicate your body was laying on a wooden deck made of 2x4s.
You might be asking yourself why a suspect would wait several hours before turning a body. One possibility is that they returned to the scene for some reason. They may have forgotten something or decided later on to come back and remove evidence like shell casings that might be used to implicate them.