Tear stains are usually caused by dye molecules called porphyrins which are iron-containing molecules, produced when the body breaks down red blood cells. Porphyrins are primarily excreted through bile and the intestinal tract, but in dogs a significant amount of porphyrin is also excreted through tears, saliva and urine.
Certain breeds of dogs are more susceptible to tear stains, or it can simply be a case of a particular dog being predisposed towards getting tear stains.
Stains appear when tears or saliva containing porphyrins sit on white fur for any length of time and, while all dogs produce porphyrins, the staining is particularly noticeable on lighter coloured hair. The stains also tend to intensify and darken when the dog has been exposed to sunlight.
Some dogs also produce more porphyrins than others, often as a result of a low level, chronic bacterial infection. In these cases, antibiotics may be required.
Other dogs have brown staining on the face, which is not a result of more usual tear staining. These can appear as a result of a yeast infection secondary to poor grooming maintenance, such as the eyes being chronically wet with tears, due to the owner not cleaning the dog's face and keeping their fur trimmed.