How can small amounts of cobalt oxide counteract the yellow color caused by iron in glass?

In the context of cobalt processing, small amounts of cobalt oxide are used to neutralize the yellow tint caused by the presence of iron in plate and window glass.1

By adding approximately 1 to 45 grams (0.04 to 1.6 ounces) of cobalt oxide per ton of glass, manufacturers can effectively counteract the unwanted yellow coloration. This process relies on the distinct purple-blue color imparted by cobalt, which balances the optical properties of the glass mixture. A similar principle is applied in the production of pottery, sanitary ware, and tiles, where cobalt oxide is used to neutralize iron-based discoloration, as well as in white enamels to achieve a similar effect.12