[header] How to prevent tear staining in dogs [title] Understand tear stains. [step] Tears contain a chemical called porphyrin that turns rust-colored when exposed to the air. A process called oxidation causes an eventual color stain to a dog's fur. 
Just like hair dye, it takes a while for the porphyrin to stain fur. Tear staining is a result of tears not being wiped away before the fur gets stained.