First, ... those suffering from porphyria look severely...

First, ... those suffering from porphyria look severely anemic because their hemoglobin is not being utilized efficiently and their blood is not as oxygenated as it should be. Second, the porphyrin rings that cannot do their job are deposited in the subcutaneous fat beneath the skin. The chemical is photosensitive, and in sunlight it can release electrons that damage the skin and may cause severe blistering -- hence the vampire's fear of sunlight.