Gel based silver staining of proteins is thought to occur by selective reduction of silver ions to insoluble metallic silver at specific initiation sites in the vicinity of the protein molecules. Silver stained protein bands generally are dark brown or black with considerable variation in color intensity. The color variation has been attributed to diffractive scattering by silver grains of different sizes. Our experiments, however, demonstrate that color variation is due to the formation of silver chromate deposits that are incorporated into formalin fixed proteins. Understanding the mechanism of silver staining is essential for developing a method for protein quantification.