White-letter tires turn brown for a few reasons. Mostly, the reason is lack of proper maintenance and cleaning. When you skip regular cleaning, dust and stains start accumulating on the white letters. Also, the material of the tire plays an important in creating the brown layer. Antizonants elements are used in the rubber tires to prevent them from drying or cracking. Sometimes these elements are released from the tire and react with oxygen. This reaction creates a brown residue that is left behind on the tire's sidewall.